threads / discuss / 31720

upload-pack is slow with lots of refs

Subject: upload-pack is slow with lots of refs

## tl;dr

24 messages between Oct 3, 2012 and Oct 9, 2012.

replies: 23people: 7as markdown or json

Ævar Arnfjörð Bjarmason· Oct 3, 2012, 12:36 UTC · lore

I'm creating a system where a lot of remotes constantly fetch from a central repository for deployment purposes, but I've noticed that even with a remote.$name.fetch configuration to only get certain refs a "git fetch" will still call git-upload pack which will provide a list of all references.

This is being done against a repository with tens of thousands of refs (it has a tag for each deployment), so it ends up burning a lot of CPU time on the uploader/receiver side.

Has there been any work on extending the protocol so that the client tells the server what refs it's interested in?

I've been looking at turning this into a push mechanism instead of a poll mechanism, but I've also noted that even when you one tag you still end up listing all refs on the remote side:

    $ GIT_TRACE=1 git push origin my-new-tag -n
    trace: built-in: git 'push' 'origin' 'my-new-tag' '-n'
    trace: run_command: 'ssh' 'avar@git.example.com' 'git-receive-pack
'\''/gitroot/example.git'\'''
    nohup: redirecting stderr to stdout
    To ssh://avar@git.example.com/gitroot/example.git
     * [new tag]         my-new-tag -> my-new-tag

Which seems like a lot of superfluous work when it presumably only needs to check if there's a remote "my-new-tag" tag which conflicts with what you're pushing..

Nguyen Thai Ngoc Duy· Oct 3, 2012, 13:06 UTC · re: Ævar Arnfjörð Bjarmason · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 3, 2012 at 7:36 PM, Ævar Arnfjörð Bjarmason <avarab@gmail.com> wrote:

Show 9 quoted lines
> I'm creating a system where a lot of remotes constantly fetch from a
> central repository for deployment purposes, but I've noticed that even
> with a remote.$name.fetch configuration to only get certain refs a
> "git fetch" will still call git-upload pack which will provide a list
> of all references.
>
> This is being done against a repository with tens of thousands of refs
> (it has a tag for each deployment), so it ends up burning a lot of CPU
> time on the uploader/receiver side.
If all refs are packed, will it still burn lots of CPU on server side?
> Has there been any work on extending the protocol so that the client
> tells the server what refs it's interested in?

It'll be a new protocol, not an extension for git protocol. Ref advertising is step 1. Capababilities are advertised much later. The client has to time to tell the server what protocol version it likes to use for step 1. (I looked at this protcol extension from a different angle. I wanted to compress the ref list for git protocol. But git over http compresses well so I don't care much.)

On that git-over-http, I don't know, maybe git client can send something as http headers, which are recognized by the server end, to negotiate interested ref patterns?

-- 
Duy
Jeff King· Oct 3, 2012, 18:03 UTC · re: Ævar Arnfjörð Bjarmason · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 03, 2012 at 02:36:00PM +0200, Ævar Arnfjörð Bjarmason wrote:
Show 9 quoted lines
> I'm creating a system where a lot of remotes constantly fetch from a
> central repository for deployment purposes, but I've noticed that even
> with a remote.$name.fetch configuration to only get certain refs a
> "git fetch" will still call git-upload pack which will provide a list
> of all references.
> 
> This is being done against a repository with tens of thousands of refs
> (it has a tag for each deployment), so it ends up burning a lot of CPU
> time on the uploader/receiver side.

Where is the CPU being burned? Are your refs packed (that's a huge savings)? What are the refs like? Are they .have refs from an alternates repository, or real refs? Are they pointing to commits or tag objects?

What version of git are you using? In the past year or so, I've made several tweaks to speed up large numbers of refs, including:

  - cff38a5 (receive-pack: eliminate duplicate .have refs, v1.7.6); note
    that this only helps if they are being pulled in by an alternates
    repo. And even then, it only helps if they are mostly duplicates;
    distinct ones are still O(n^2).
  - 7db8d53 (fetch-pack: avoid quadratic behavior in remove_duplicates)
    a0de288 (fetch-pack: avoid quadratic loop in filter_refs)
    Both in v1.7.11. I think there is still a potential quadratic loop
    in mark_complete()
  - 90108a2 (upload-pack: avoid parsing tag destinations)
    926f1dd (upload-pack: avoid parsing objects during ref advertisement)
    Both in v1.7.10. Note that tag objects are more expensive to
    advertise than commits, because we have to load and peel them.

Even with those patches, though, I found that it was something like ~2s to advertise 100,000 refs.

> Has there been any work on extending the protocol so that the client
> tells the server what refs it's interested in?

I don't think so. It would be hard to do in a backwards-compatible way, because the advertisement is the first thing the server says, before it has negotiated any capabilities with the client at all.

-Peff
Junio C Hamano· Oct 3, 2012, 18:53 UTC · re: Jeff King · lore

Re: upload-pack is slow with lots of refs

Jeff King <peff@peff.net> writes:
Show 6 quoted lines
>> Has there been any work on extending the protocol so that the client
>> tells the server what refs it's interested in?
>
> I don't think so. It would be hard to do in a backwards-compatible way,
> because the advertisement is the first thing the server says, before it
> has negotiated any capabilities with the client at all.
That is being discussed but hasn't surfaced on the list.
Jeff King· Oct 3, 2012, 18:55 UTC · re: Junio C Hamano · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 03, 2012 at 11:53:35AM -0700, Junio C Hamano wrote:
Show 10 quoted lines
> Jeff King <peff@peff.net> writes:
> 
> >> Has there been any work on extending the protocol so that the client
> >> tells the server what refs it's interested in?
> >
> > I don't think so. It would be hard to do in a backwards-compatible way,
> > because the advertisement is the first thing the server says, before it
> > has negotiated any capabilities with the client at all.
> 
> That is being discussed but hasn't surfaced on the list.

Out of curiosity, how are you thinking about triggering such a new behavior in a backwards-compatible way? Invoke git-upload-pack2, and fall back to reconnecting to start git-upload-pack if it fails?

-Peff
Shawn Pearce· Oct 3, 2012, 19:41 UTC · re: Jeff King · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 3, 2012 at 11:55 AM, Jeff King <peff@peff.net> wrote:
Show 15 quoted lines
> On Wed, Oct 03, 2012 at 11:53:35AM -0700, Junio C Hamano wrote:
>> Jeff King <peff@peff.net> writes:
>>
>> >> Has there been any work on extending the protocol so that the client
>> >> tells the server what refs it's interested in?
>> >
>> > I don't think so. It would be hard to do in a backwards-compatible way,
>> > because the advertisement is the first thing the server says, before it
>> > has negotiated any capabilities with the client at all.
>>
>> That is being discussed but hasn't surfaced on the list.
>
> Out of curiosity, how are you thinking about triggering such a new
> behavior in a backwards-compatible way? Invoke git-upload-pack2, and
> fall back to reconnecting to start git-upload-pack if it fails?

Basically, yes. New clients connect for git-upload-pack2. Over git:// the remote peer will just close the TCP socket with no messages. The client can fallback to git-upload-pack and try again. Over SSH a similar thing will happen in the sense there is no data output from the remote side, so the client can try again. This has the downside of authentication twice over SSH, which may prompt for a password twice. But the user can get out of this by setting remote.NAME.uploadpack = git-upload-pack and thus force the Git client to use the current protocol if they have a new client and must continue to work over SSH with an old server, and don't use an ssh-agent.

Over HTTP we can request ?service=git-upload-pack2 and retry just like git:// would, or be a bit smarter and say ?service=git-upload-pack&v=2, and determine the protocol support of the remote peer based on the response we get. If we see an immediate advertisement its still the "v1" protocol, if we get back the "yes I speak v2" response like git:// would see, we can continue the conversation from there.

Jeff King· Oct 3, 2012, 20:13 UTC · re: Shawn Pearce · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 03, 2012 at 12:41:38PM -0700, Shawn O. Pearce wrote:
Show 14 quoted lines
> > Out of curiosity, how are you thinking about triggering such a new
> > behavior in a backwards-compatible way? Invoke git-upload-pack2, and
> > fall back to reconnecting to start git-upload-pack if it fails?
> 
> Basically, yes. New clients connect for git-upload-pack2. Over git://
> the remote peer will just close the TCP socket with no messages. The
> client can fallback to git-upload-pack and try again. Over SSH a
> similar thing will happen in the sense there is no data output from
> the remote side, so the client can try again. This has the downside of
> authentication twice over SSH, which may prompt for a password twice.
> But the user can get out of this by setting remote.NAME.uploadpack =
> git-upload-pack and thus force the Git client to use the current
> protocol if they have a new client and must continue to work over SSH
> with an old server, and don't use an ssh-agent.

It's a shame that we have to reestablish the TCP or ssh connection to do the retry. The password thing is annoying, but also it just wastes a round-trip. It means we'd probably want to default the v2 probe to off (and let the user turn it on for a specific remote) until v2 is much more common than v1. Otherwise everyone pays the price.

It may also be worth designing v2 to handle more graceful capability negotiation so this doesn't come up again.

Another alternative would be to tweak git-daemon to allow more graceful fallback. That wouldn't help us now, but it would if we ever wanted a v3. For stock ssh, you could send:

  sh -c 'git upload-pack2; test $? = 127 && git-upload-pack'

which would work if you have an unrestricted shell on the other side. But it would break for a restricted shell or other "fake" ssh environment. It's probably too ugly to have restricted shells recognize that as a magic token (well, I could maybe even live with the ugliness, but it is not strictly backwards compatible).

I was hoping we could do something like "git upload-pack --v2", but I'm pretty sure current git-daemon would reject that.

Show 7 quoted lines
> Over HTTP we can request ?service=git-upload-pack2 and retry just like
> git:// would, or be a bit smarter and say
> ?service=git-upload-pack&v=2, and determine the protocol support of
> the remote peer based on the response we get. If we see an immediate
> advertisement its still the "v1" protocol, if we get back the "yes I
> speak v2" response like git:// would see, we can continue the
> conversation from there.

Yeah, I would think "&v=2" would be better simply to avoid the round-trip if we fail. It should be safe to turn the new protocol on by default for http, then.

-Peff
Sascha Cunz· Oct 4, 2012, 21:52 UTC · re: Jeff King · lore

Re: upload-pack is slow with lots of refs

Am Mittwoch, 3. Oktober 2012, 16:13:16 schrieb Jeff King:
Show 21 quoted lines
> On Wed, Oct 03, 2012 at 12:41:38PM -0700, Shawn O. Pearce wrote:
> > > Out of curiosity, how are you thinking about triggering such a new
> > > behavior in a backwards-compatible way? Invoke git-upload-pack2, and
> > > fall back to reconnecting to start git-upload-pack if it fails?
> > 
> > Basically, yes. New clients connect for git-upload-pack2. Over git://
> > the remote peer will just close the TCP socket with no messages. The
> > client can fallback to git-upload-pack and try again. Over SSH a
> > similar thing will happen in the sense there is no data output from
> > the remote side, so the client can try again. This has the downside of
> > authentication twice over SSH, which may prompt for a password twice.
> > But the user can get out of this by setting remote.NAME.uploadpack =
> > git-upload-pack and thus force the Git client to use the current
> > protocol if they have a new client and must continue to work over SSH
> > with an old server, and don't use an ssh-agent.
> 
> It's a shame that we have to reestablish the TCP or ssh connection to do
> the retry. The password thing is annoying, but also it just wastes a
> round-trip. It means we'd probably want to default the v2 probe to off
> (and let the user turn it on for a specific remote) until v2 is much
> more common than v1. Otherwise everyone pays the price.
Would it be possible to use this workflow:
- Every client connects per default to v1
- If server is capable of v2, it sends a flag along with the usual response
  (A v1 server will obviously not send that flag)
- If client is also capable of v2 and gets the flag, it enables v2 for
  just that remote (probably unless the user said, "i never want to")
- Next time the client connects to that remote it will use v2.

I'm not sure, if this is possible, since I think to remember that I have read in the Documentation folder something along the line: Capabilities announced from the server mean "I want you to use exactly these flags".

Sascha
Jeff King· Oct 5, 2012, 00:20 UTC · re: Sascha Cunz · lore

Re: upload-pack is slow with lots of refs

On Thu, Oct 04, 2012 at 11:52:13PM +0200, Sascha Cunz wrote:
Show 6 quoted lines
> Would it be possible to use this workflow:
> 
> - Every client connects per default to v1
> 
> - If server is capable of v2, it sends a flag along with the usual response
>   (A v1 server will obviously not send that flag)

That is more or less the strategy we use for existing extensions (your "flag" is a space-separated list of capability strings). But in this case, the idea would be to change what the "usual response" is. Since a v1 client would be expecting the response, we must send it, but at that point it is too late to make the change. So we need to see some flag from the client before the server says anything.

And the problem is that the client sending that flag will break v1 servers, and the client would need to waste time doing a retry when connecting to the (initially more common) v1 servers.

> - If client is also capable of v2 and gets the flag, it enables v2 for
>   just that remote (probably unless the user said, "i never want to")
> 
> - Next time the client connects to that remote it will use v2.

So yeah, that would work to help with the wasted time. We'd have git-upload-pack2 to do the v2 protocol, but the v1 git-upload-pack for the server would say "by the way, next time you connect, try v2 first". So the client would have to store a version number for each remote. Which is not too onerous.

Another way to think of it is phasing it in like this:
  1. Add v2 support to client and server. Initially, clients try only
     v1.
  2. Add a remote.*.preferProtocol config option, defaulting to v1. This
     lets people turn on v2 for remotes they know support it. If v2
     fails, still fall back to v1.
  3. Add a server upload-pack capability that says "by the way, try v2
     next time".  Have the client set the preferProtocol config option
     for a remote if we see that capability.
  4. Wait a while until v2 is very popular.
  5. Switch the default for preferProtocol to v2 (but still fall back to
     v1).

So always fall back and remain compatible, and let the config option just be an optimization to avoid extra failed requests.

> I'm not sure, if this is possible, since I think to remember that I have read 
> in the Documentation folder something along the line: Capabilities announced 
> from the server mean "I want you to use exactly these flags".

No, the server capability says "I can do this", and the client should respond with "I want you to do this". Because the server might be talking to an older client that does not know what "this" is, it must handle the case that the capability does not come back.

-Peff
Johannes Sixt· Oct 5, 2012, 06:24 UTC · re: Shawn Pearce · lore

Re: upload-pack is slow with lots of refs

Am 10/3/2012 21:41, schrieb Shawn Pearce:
Show 22 quoted lines
> On Wed, Oct 3, 2012 at 11:55 AM, Jeff King <peff@peff.net> wrote:
>> On Wed, Oct 03, 2012 at 11:53:35AM -0700, Junio C Hamano wrote:
>>> Jeff King <peff@peff.net> writes:
>>>
>>>>> Has there been any work on extending the protocol so that the client
>>>>> tells the server what refs it's interested in?
>>>>
>>>> I don't think so. It would be hard to do in a backwards-compatible way,
>>>> because the advertisement is the first thing the server says, before it
>>>> has negotiated any capabilities with the client at all.
>>>
>>> That is being discussed but hasn't surfaced on the list.
>>
>> Out of curiosity, how are you thinking about triggering such a new
>> behavior in a backwards-compatible way? Invoke git-upload-pack2, and
>> fall back to reconnecting to start git-upload-pack if it fails?
> 
> Basically, yes. New clients connect for git-upload-pack2. Over git://
> the remote peer will just close the TCP socket with no messages. The
> client can fallback to git-upload-pack and try again. Over SSH a
> similar thing will happen in the sense there is no data output from
> the remote side, so the client can try again.

These connections are bidirectional. Upload-pack can just start advertising refs in the "v1" way and announce a "v2" capability and listen for response in parallel. A v2 capable client can start sending "wants" or some other signal as soon as it sees the "v2" capability. Upload-pack, which was listening for responses in parallel, can interrupt its advertisements and continue with v2 protocol from here.

This sounds so simple (not the implementation, of course) - I must be missing something.

-- Hannes
Shawn Pearce· Oct 5, 2012, 16:57 UTC · re: Johannes Sixt · lore

Re: upload-pack is slow with lots of refs

On Thu, Oct 4, 2012 at 11:24 PM, Johannes Sixt <j.sixt@viscovery.net> wrote:
Show 25 quoted lines
> Am 10/3/2012 21:41, schrieb Shawn Pearce:
>> On Wed, Oct 3, 2012 at 11:55 AM, Jeff King <peff@peff.net> wrote:
>>> On Wed, Oct 03, 2012 at 11:53:35AM -0700, Junio C Hamano wrote:
>>>> Jeff King <peff@peff.net> writes:
>>>>
>>>>>> Has there been any work on extending the protocol so that the client
>>>>>> tells the server what refs it's interested in?
>>>>>
>>>>> I don't think so. It would be hard to do in a backwards-compatible way,
>>>>> because the advertisement is the first thing the server says, before it
>>>>> has negotiated any capabilities with the client at all.
>>>>
>>>> That is being discussed but hasn't surfaced on the list.
>>>
>>> Out of curiosity, how are you thinking about triggering such a new
>>> behavior in a backwards-compatible way? Invoke git-upload-pack2, and
>>> fall back to reconnecting to start git-upload-pack if it fails?
>>
>> Basically, yes. New clients connect for git-upload-pack2. Over git://
>> the remote peer will just close the TCP socket with no messages. The
>> client can fallback to git-upload-pack and try again. Over SSH a
>> similar thing will happen in the sense there is no data output from
>> the remote side, so the client can try again.
>
> These connections are bidirectional.
Smart HTTP is not bidirectional.
Show 9 quoted lines
> Upload-pack can just start
> advertising refs in the "v1" way and announce a "v2" capability and listen
> for response in parallel. A v2 capable client can start sending "wants" or
> some other signal as soon as it sees the "v2" capability. Upload-pack,
> which was listening for responses in parallel, can interrupt its
> advertisements and continue with v2 protocol from here.
>
> This sounds so simple (not the implementation, of course) - I must be
> missing something.

Smart HTTP is not bidirectional. The client can't cut off the server. Its also more complex to code the server to listen for a stop command from the client at the same time the server is blasting out useless references to the client.

Johannes Sixt· Oct 8, 2012, 15:05 UTC · re: Shawn Pearce · lore

Re: upload-pack is slow with lots of refs

Am 05.10.2012 18:57, schrieb Shawn Pearce:
Show 12 quoted lines
> On Thu, Oct 4, 2012 at 11:24 PM, Johannes Sixt <j.sixt@viscovery.net> wrote:
>> Upload-pack can just start
>> advertising refs in the "v1" way and announce a "v2" capability and listen
>> for response in parallel. A v2 capable client can start sending "wants" or
>> some other signal as soon as it sees the "v2" capability. Upload-pack,
>> which was listening for responses in parallel, can interrupt its
>> advertisements and continue with v2 protocol from here.
>>
>> This sounds so simple (not the implementation, of course) - I must be
>> missing something.
> 
> Smart HTTP is not bidirectional. The client can't cut off the server.

Smart HTTP does not need it: you already posted a better solution (I'm refering to "&v=2").

> Its also more complex to code the server to listen for a stop command
> from the client at the same time the server is blasting out useless
> references to the client.

At least the server side does not seem to be that complex. See below. Of course, the server blasted out some refs, but I'm confident that in practice the client will be able to signal v2 capability after a few packets of advertisements. You can switch on TCP_NODELAY for the first line with the capabilities to ensure it goes out on the wire ASAP.

diff --git a/upload-pack.c b/upload-pack.c
index 2e90ccb..c29ae04 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -720,11 +720,20 @@ static void receive_needs(void)
 	free(shallows.objects);
 }
 
+static int client_spoke(void)
+{
+	struct pollfd pfd;
+	pfd.fd = 0;
+	pfd.events = POLLIN;
+	return poll(&pfd, 1, 0) > 0 &&
+		(pfd.revents & (POLLIN|POLLHUP));
+}
+
 static int send_ref(const char *refname, const unsigned char *sha1, int flag, void *cb_data)
 {
 	static const char *capabilities = "multi_ack thin-pack side-band"
 		" side-band-64k ofs-delta shallow no-progress"
-		" include-tag multi_ack_detailed";
+		" include-tag multi_ack_detailed version2";
 	struct object *o = lookup_unknown_object(sha1);
 	const char *refname_nons = strip_namespace(refname);
 
@@ -752,7 +761,8 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 		if (o)
 			packet_write(1, "%s %s^{}\n", sha1_to_hex(o->sha1), refname_nons);
 	}
-	return 0;
+
+	return client_spoke();
 }
 
 static int mark_our_ref(const char *refname, const unsigned char *sha1, int flag, void *cb_data)
@@ -771,8 +781,14 @@ static void upload_pack(void)
 {
 	if (advertise_refs || !stateless_rpc) {
 		reset_timeout();
-		head_ref_namespaced(send_ref, NULL);
-		for_each_namespaced_ref(send_ref, NULL);
+		if (head_ref_namespaced(send_ref, NULL) ||
+		    for_each_namespaced_ref(send_ref, NULL)) {
+			/*
+			 * TODO: continue with protocol version 2
+			 * optimization: do not send refs
+			 * that were already sent
+			 */
+		}
 		packet_flush(1);
 	} else {
 		head_ref_namespaced(mark_our_ref, NULL);
Shawn Pearce· Oct 9, 2012, 06:46 UTC · re: Johannes Sixt · lore

Re: upload-pack is slow with lots of refs

On Mon, Oct 8, 2012 at 8:05 AM, Johannes Sixt <j6t@kdbg.org> wrote:
Show 16 quoted lines
> Am 05.10.2012 18:57, schrieb Shawn Pearce:
>> On Thu, Oct 4, 2012 at 11:24 PM, Johannes Sixt <j.sixt@viscovery.net> wrote:
>>> Upload-pack can just start
>>> advertising refs in the "v1" way and announce a "v2" capability and listen
>>> for response in parallel. A v2 capable client can start sending "wants" or
>>> some other signal as soon as it sees the "v2" capability. Upload-pack,
>>> which was listening for responses in parallel, can interrupt its
>>> advertisements and continue with v2 protocol from here.
>>>
>>> This sounds so simple (not the implementation, of course) - I must be
>>> missing something.
>>
>> Smart HTTP is not bidirectional. The client can't cut off the server.
>
> Smart HTTP does not need it: you already posted a better solution (I'm
> refering to "&v=2").
Yes but then it diverges even further from the native bidirectional protocol.
Show 9 quoted lines
>> Its also more complex to code the server to listen for a stop command
>> from the client at the same time the server is blasting out useless
>> references to the client.
>
> At least the server side does not seem to be that complex. See below.
> Of course, the server blasted out some refs, but I'm confident that in
> practice the client will be able to signal v2 capability after a few packets
> of advertisements. You can switch on TCP_NODELAY for the first line with
> the capabilities to ensure it goes out on the wire ASAP.
...
Show 7 quoted lines
> +static int client_spoke(void)
> +{
> +       struct pollfd pfd;
> +       pfd.fd = 0;
> +       pfd.events = POLLIN;
> +       return poll(&pfd, 1, 0) > 0 &&
> +               (pfd.revents & (POLLIN|POLLHUP));

Except doing this in Java is harder on an arbitrary InputStream type. I guess we really only care about basic TCP, in which case we can use NIO to implement an emulation of poll, and SSH, where MINA SSHD probably doesn't provide a way to see if the client has given us data without blocking. That makes supporting v2 really hard in e.g. Gerrit Code Review. You could argue that its improper to attempt to implement a network protocol in a language whose standard libraries have gone out of their way to prevent you from polling to see if data is immediately available, but I prefer to ignore such arguments.

As it turns out we don't really have this problem with git://. Clients can bury a v2 request in the extended headers where the host line appears today. Its a bit tricky because of that \0 bug causing infinite looping, but IIRC using \0\0 is safe even against ancient servers. So git:// and http:// both have a way where the client can ask for v2 support before the server speaks, and have it transparently be ignored by ancient servers.

The only place we have a problem is SSH. That exec of the remote binary is just super-strict. Its good to be paranoid, but its also locked out any chance we have at doing the upgrade over SSH without having to run two SSH commands in the worst case. I guess the best approach is to try the v1 protocol by default, have the remote advertise it supports v2, and remember this on a per-host basis in ~/.gitconfig for future requests. Users could always force a specific preference with remote.NAME.uploadpack variable or --uploadpack command line flag.

Johannes Sixt· Oct 9, 2012, 20:30 UTC · re: Shawn Pearce · lore

Re: upload-pack is slow with lots of refs

Am 09.10.2012 08:46, schrieb Shawn Pearce:
Show 8 quoted lines
> On Mon, Oct 8, 2012 at 8:05 AM, Johannes Sixt <j6t@kdbg.org> wrote:
>> Am 05.10.2012 18:57, schrieb Shawn Pearce:
>>> Smart HTTP is not bidirectional. The client can't cut off the server.
>>
>> Smart HTTP does not need it: you already posted a better solution (I'm
>> refering to "&v=2").
> 
> Yes but then it diverges even further from the native bidirectional protocol.

I won't argue here because I know next to nothing about Smart HTTP. But it sounds like you either have compatibility, but a diverging protocol or at least implementation, or no compatibility.

Show 17 quoted lines
>> +static int client_spoke(void)
>> +{
>> +       struct pollfd pfd;
>> +       pfd.fd = 0;
>> +       pfd.events = POLLIN;
>> +       return poll(&pfd, 1, 0) > 0 &&
>> +               (pfd.revents & (POLLIN|POLLHUP));
> 
> Except doing this in Java is harder on an arbitrary InputStream type.
> I guess we really only care about basic TCP, in which case we can use
> NIO to implement an emulation of poll, and SSH, where MINA SSHD
> probably doesn't provide a way to see if the client has given us data
> without blocking. That makes supporting v2 really hard in e.g. Gerrit
> Code Review. You could argue that its improper to attempt to implement
> a network protocol in a language whose standard libraries have gone
> out of their way to prevent you from polling to see if data is
> immediately available, but I prefer to ignore such arguments.

Can't you read the inbound stream in a second thread while the first thread writes the advertisements to the outbound stream? Then you don't even need to poll; you can just read the 4-byte length header, stash it away and set a flag. The implementation of client_spoke() would only amount to check that flag.

> As it turns out we don't really have this problem with git://. Clients
> can bury a v2 request in the extended headers where the host line
> appears today.

I tried, but it seems that todays git-daemons are too strict and accept only \0host=foo\0, nothing else :-(

-- Hannes
Johannes Sixt· Oct 9, 2012, 20:46 UTC · re: Johannes Sixt · lore

Re: upload-pack is slow with lots of refs

Am 09.10.2012 22:30, schrieb Johannes Sixt:
Show 7 quoted lines
> Am 09.10.2012 08:46, schrieb Shawn Pearce:
>> As it turns out we don't really have this problem with git://. Clients
>> can bury a v2 request in the extended headers where the host line
>> appears today.
> 
> I tried, but it seems that todays git-daemons are too strict and accept
> only \0host=foo\0, nothing else :-(

I take that back: Modern git-daemons accept "\0host=foo\0\0version=2\0", as you said.

It looks like SSH is the only stubborn protocol.
-- Hannes
Ævar Arnfjörð Bjarmason· Oct 3, 2012, 20:16 UTC · re: Jeff King · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 3, 2012 at 8:03 PM, Jeff King <peff@peff.net> wrote:
Show 36 quoted lines
> On Wed, Oct 03, 2012 at 02:36:00PM +0200, Ævar Arnfjörð Bjarmason wrote:
>
>> I'm creating a system where a lot of remotes constantly fetch from a
>> central repository for deployment purposes, but I've noticed that even
>> with a remote.$name.fetch configuration to only get certain refs a
>> "git fetch" will still call git-upload pack which will provide a list
>> of all references.
>>
>> This is being done against a repository with tens of thousands of refs
>> (it has a tag for each deployment), so it ends up burning a lot of CPU
>> time on the uploader/receiver side.
>
> Where is the CPU being burned? Are your refs packed (that's a huge
> savings)? What are the refs like? Are they .have refs from an alternates
> repository, or real refs? Are they pointing to commits or tag objects?
>
> What version of git are you using?  In the past year or so, I've made
> several tweaks to speed up large numbers of refs, including:
>
>   - cff38a5 (receive-pack: eliminate duplicate .have refs, v1.7.6); note
>     that this only helps if they are being pulled in by an alternates
>     repo. And even then, it only helps if they are mostly duplicates;
>     distinct ones are still O(n^2).
>
>   - 7db8d53 (fetch-pack: avoid quadratic behavior in remove_duplicates)
>     a0de288 (fetch-pack: avoid quadratic loop in filter_refs)
>     Both in v1.7.11. I think there is still a potential quadratic loop
>     in mark_complete()
>
>   - 90108a2 (upload-pack: avoid parsing tag destinations)
>     926f1dd (upload-pack: avoid parsing objects during ref advertisement)
>     Both in v1.7.10. Note that tag objects are more expensive to
>     advertise than commits, because we have to load and peel them.
>
> Even with those patches, though, I found that it was something like ~2s
> to advertise 100,000 refs.

I can't provide all the details now (not with access to that machine now), but briefly:

 * The git client/server version is 1.7.8
 * The repository has around 50k refs, they're "real" refs, almost all
   of them (say all but 0.5k-1k) are annotated tags, the rest are
   branches.
 * >99% of them are packed, there's a weekly cronjob that packs them
   all up, there were a few newly pushed branches and tags outside of
   the
 * I tried "echo -n | git upload-pack <repo>" on both that 50k
   repository and a repository with <100 refs, the former took around
   ~1-2s to run on a 24 core box and the latter ~500ms.
 * When I ran git-upload-pack with GNU parallel I managed around 20/s
   packs on the 24 core box on the 50k ref one, 40/s on the 100 ref
   one.
 * A co-worker who was working on this today tried it on 1.7.12 and
   claimed that it had the same performance characteristics.
 * I tried to profile it under gcc -pg && echo -n | ./git-upload-pack
   <repo> but it doesn't produce a profile like that, presumably
   because the process exits unsuccessfully.
   Maybe someone here knows offhand what mock data I could feed
   git-upload-pack to make it happy to just list the refs, or better
   yet do a bit more work which it would do if it were actually doing
   the fetch (I suppose I could just do a fetch, but I wanted to do
   this from a locally compiled checkout).
Show 6 quoted lines
>> Has there been any work on extending the protocol so that the client
>> tells the server what refs it's interested in?
>
> I don't think so. It would be hard to do in a backwards-compatible way,
> because the advertisement is the first thing the server says, before it
> has negotiated any capabilities with the client at all.
I suppose at least for the ssh protocol we could just do:
    ssh server "(git upload-pack <repo> --refs=* || git upload-pack <repo>)"

And something similar with HTTP headers, but that of course leaves the git:// protocol.

Jeff King· Oct 3, 2012, 21:20 UTC · re: Ævar Arnfjörð Bjarmason · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 03, 2012 at 10:16:56PM +0200, Ævar Arnfjörð Bjarmason wrote:
Show 8 quoted lines
> I can't provide all the details now (not with access to that machine
> now), but briefly:
> 
>  * The git client/server version is 1.7.8
> 
>  * The repository has around 50k refs, they're "real" refs, almost all
>    of them (say all but 0.5k-1k) are annotated tags, the rest are
>    branches.

I'd definitely try upgrading, then; I got measurable speedups from this exact case using the patches in v1.7.10.

>  * >99% of them are packed, there's a weekly cronjob that packs them
>    all up, there were a few newly pushed branches and tags outside of
>    the

A few strays shouldn't make a big difference. The killer is calling open(2) 50,000 times, but having most of it packed should prevent that. I suspect Michael Haggerty's work on the ref cache may help, too (otherwise we have to try each packed ref in the filesystem to make sure nobody has written it since we packed).

>  * I tried "echo -n | git upload-pack <repo>" on both that 50k
>    repository and a repository with <100 refs, the former took around
>    ~1-2s to run on a 24 core box and the latter ~500ms.
More cores won't help, of course, as dumping the refs is single-threaded.

With v1.7.12, my ~400K test repository takes about 0.8s to run (on my 2-year-old 1.8 GHz i7, though it is probably turbo-boosting to 3 GHz). So I'm surprised it is so slow.

Your 100-ref case is slow, too. Upload-pack's initial advertisement on my linux-2.6 repository (without about 900 refs) is more like 20ms. I'd

>  * A co-worker who was working on this today tried it on 1.7.12 and
>    claimed that it had the same performance characteristics.
That's surprising to me. Can you try to verify those numbers?
>  * I tried to profile it under gcc -pg && echo -n | ./git-upload-pack
>    <repo> but it doesn't produce a profile like that, presumably
>    because the process exits unsuccessfully.

If it's a recent version of Linux, you'll get much nicer results with perf. Here's what my 400K-ref case looks like:

  $ time echo 0000 | perf record git-upload-pack . >/dev/null
  real    0m0.808s
  user    0m0.660s
  sys     0m0.136s
  $ perf report | grep -v ^# | head
  11.40%  git-upload-pack  libc-2.13.so        [.] vfprintf
   9.70%  git-upload-pack  git-upload-pack     [.] find_pack_entry_one
   7.64%  git-upload-pack  git-upload-pack     [.] check_refname_format
   6.81%  git-upload-pack  libc-2.13.so        [.] __memcmp_sse4_1
   5.79%  git-upload-pack  libc-2.13.so        [.] getenv
   4.20%  git-upload-pack  libc-2.13.so        [.] __strlen_sse42
   3.72%  git-upload-pack  git-upload-pack     [.] ref_entry_cmp_sslice
   3.15%  git-upload-pack  git-upload-pack     [.] read_packed_refs
   2.65%  git-upload-pack  git-upload-pack     [.] sha1_to_hex
   2.44%  git-upload-pack  libc-2.13.so        [.] _IO_default_xsputn

So nothing too surprising, though there is some room for improvement (e.g., it looks like we are calling getenv in a tight loop, which could be hoisted out to a single call).

Do note that this version of git was compiled with -O3. Compiling with -O0 produces very different results (it's more like 1.3s, and the hotspots are check_refname_component and sha1_to_hex).

Show 5 quoted lines
>    Maybe someone here knows offhand what mock data I could feed
>    git-upload-pack to make it happy to just list the refs, or better
>    yet do a bit more work which it would do if it were actually doing
>    the fetch (I suppose I could just do a fetch, but I wanted to do
>    this from a locally compiled checkout).

If you feed "0000" as I did above, that is the flush signal for "I have no more lines to send you", which means that we are not actually fetching anything. I.e., this is the exact same conversation a no-op "git fetch" would produce.

-Peff
Ævar Arnfjörð Bjarmason· Oct 3, 2012, 22:15 UTC · re: Jeff King · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 3, 2012 at 11:20 PM, Jeff King <peff@peff.net> wrote:
Thanks for all that info, it's really useful.
>>  * A co-worker who was working on this today tried it on 1.7.12 and
>>    claimed that it had the same performance characteristics.
>
> That's surprising to me. Can you try to verify those numbers?

I think he was wrong, I tested this on git.git by first creating a lot of tags:

     parallel --eta "git tag -a -m"{}" test-again-{}" ::: $(git rev-list HEAD)
Then doing:
    git pack-refs --all
    git repack -A -d

And compiled with -g -O3 I get around 1.55 runs/s of git-upload-pack on 1.7.8 and 2.59/s on the master branch.

Show 23 quoted lines
>>  * I tried to profile it under gcc -pg && echo -n | ./git-upload-pack
>>    <repo> but it doesn't produce a profile like that, presumably
>>    because the process exits unsuccessfully.
>
> If it's a recent version of Linux, you'll get much nicer results with
> perf. Here's what my 400K-ref case looks like:
>
>   $ time echo 0000 | perf record git-upload-pack . >/dev/null
>   real    0m0.808s
>   user    0m0.660s
>   sys     0m0.136s
>
>   $ perf report | grep -v ^# | head
>   11.40%  git-upload-pack  libc-2.13.so        [.] vfprintf
>    9.70%  git-upload-pack  git-upload-pack     [.] find_pack_entry_one
>    7.64%  git-upload-pack  git-upload-pack     [.] check_refname_format
>    6.81%  git-upload-pack  libc-2.13.so        [.] __memcmp_sse4_1
>    5.79%  git-upload-pack  libc-2.13.so        [.] getenv
>    4.20%  git-upload-pack  libc-2.13.so        [.] __strlen_sse42
>    3.72%  git-upload-pack  git-upload-pack     [.] ref_entry_cmp_sslice
>    3.15%  git-upload-pack  git-upload-pack     [.] read_packed_refs
>    2.65%  git-upload-pack  git-upload-pack     [.] sha1_to_hex
>    2.44%  git-upload-pack  libc-2.13.so        [.] _IO_default_xsputn
FWIW here are my results on the above pathological git.git
    $ uname -r; perf --version; echo 0000 | perf record
./git-upload-pack .>/dev/null; perf report | grep -v ^# | head
    3.2.0-2-amd64
    perf version 3.2.17
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.026 MB perf.data (~1131 samples) ]
        29.08%  git-upload-pack  libz.so.1.2.7       [.] inflate
        17.99%  git-upload-pack  libz.so.1.2.7       [.] 0xaec1
         6.21%  git-upload-pack  libc-2.13.so        [.] 0x117503
         5.69%  git-upload-pack  libcrypto.so.1.0.0  [.] 0x82c3d
         4.87%  git-upload-pack  git-upload-pack     [.] find_pack_entry_one
         3.18%  git-upload-pack  ld-2.13.so          [.] 0x886e
         2.96%  git-upload-pack  libc-2.13.so        [.] vfprintf
         2.83%  git-upload-pack  git-upload-pack     [.] search_for_subdir
         1.56%  git-upload-pack  [kernel.kallsyms]   [k] do_raw_spin_lock
         1.36%  git-upload-pack  libc-2.13.so        [.] vsnprintf

I wonder why your report doesn't note any time in libz. This is on Debian testing, maybe your OS uses different strip settings so it doesn't show up?

    $ ldd -r ./git-upload-pack
            linux-vdso.so.1 =>  (0x00007fff621ff000)
            libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f768feee000)
            libcrypto.so.1.0.0 =>
/usr/lib/x86_64-linux-gnu/libcrypto.so.1.0.0 (0x00007f768fb0a000)
            libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0
(0x00007f768f8ed000)
            libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f768f566000)
            libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f768f362000)
            /lib64/ld-linux-x86-64.so.2 (0x00007f7690117000
Jeff King· Oct 3, 2012, 23:15 UTC · re: Ævar Arnfjörð Bjarmason · lore

Re: upload-pack is slow with lots of refs

On Thu, Oct 04, 2012 at 12:15:47AM +0200, Ævar Arnfjörð Bjarmason wrote:
Show 12 quoted lines
> I think he was wrong, I tested this on git.git by first creating a lot
> of tags:
> 
>      parallel --eta "git tag -a -m"{}" test-again-{}" ::: $(git rev-list HEAD)
> 
> Then doing:
> 
>     git pack-refs --all
>     git repack -A -d
> 
> And compiled with -g -O3 I get around 1.55 runs/s of git-upload-pack
> on 1.7.8 and 2.59/s on the master branch.
Thanks for the update, that's more like what I expected.
Show 22 quoted lines
> FWIW here are my results on the above pathological git.git
> 
>     $ uname -r; perf --version; echo 0000 | perf record
> ./git-upload-pack .>/dev/null; perf report | grep -v ^# | head
>     3.2.0-2-amd64
>     perf version 3.2.17
>     [ perf record: Woken up 1 times to write data ]
>     [ perf record: Captured and wrote 0.026 MB perf.data (~1131 samples) ]
>         29.08%  git-upload-pack  libz.so.1.2.7       [.] inflate
>         17.99%  git-upload-pack  libz.so.1.2.7       [.] 0xaec1
>          6.21%  git-upload-pack  libc-2.13.so        [.] 0x117503
>          5.69%  git-upload-pack  libcrypto.so.1.0.0  [.] 0x82c3d
>          4.87%  git-upload-pack  git-upload-pack     [.] find_pack_entry_one
>          3.18%  git-upload-pack  ld-2.13.so          [.] 0x886e
>          2.96%  git-upload-pack  libc-2.13.so        [.] vfprintf
>          2.83%  git-upload-pack  git-upload-pack     [.] search_for_subdir
>          1.56%  git-upload-pack  [kernel.kallsyms]   [k] do_raw_spin_lock
>          1.36%  git-upload-pack  libc-2.13.so        [.] vsnprintf
> 
> I wonder why your report doesn't note any time in libz. This is on
> Debian testing, maybe your OS uses different strip settings so it
> doesn't show up?

Mine was on Debian unstable. The difference is probably that I have 400K refs, but only 12K unique ones (this is the master alternates repo containing every ref from every fork of rails/rails on GitHub). So I spend proportionally more time fiddling with refs and outputting than I do actually inflating tag objects.

Hmm. It seems like we should not need to open the tags at all. The main reason is to produce the "peeled" advertisement just after it. But for a packed ref with a modern version of git that supports the "peeled" extension, we should already have that information.

The hack-ish patch below tries to reuse that. The interface is terrible, and we should probably just pass the peel information via for_each_ref (peel_ref tries to do a similar thing, but it also has a bad interface; if we don't have the information already, it will redo the ref lookup. We could probably get away with a peel_sha1 which uses the same optimization trick as peel_ref).

With this patch my 800ms upload-pack drops to 600ms. I suspect it will have an even greater impact for you, since you are spending much more of your time on object loading than I am.

And note of course that while these micro-optimizations are neat, we're still going to end up shipping quite a lot of data over the wire. Moving to a protocol where we are advertising fewer refs would solve a lot more problems in the long run.

---
diff --git a/refs.c b/refs.c
index 551a0f9..68eca3a 100644
--- a/refs.c
+++ b/refs.c
@@ -510,6 +510,14 @@ static struct ref_entry *current_ref;
 
 static struct ref_entry *current_ref;
 
+/* XXX horrible interface due to implied argument. not for real use */
+const unsigned char *peel_current_ref(void)
+{
+	if (!current_ref || !(current_ref->flag & REF_KNOWS_PEELED))
+		return NULL;
+	return current_ref->u.value.peeled;
+}
+
 static int do_one_ref(const char *base, each_ref_fn fn, int trim,
 		      int flags, void *cb_data, struct ref_entry *entry)
 {
diff --git a/refs.h b/refs.h
index 9d14558..88c5445 100644
--- a/refs.h
+++ b/refs.h
@@ -14,6 +14,8 @@ struct ref_lock {
 #define REF_ISPACKED 0x02
 #define REF_ISBROKEN 0x04
 
+const unsigned char *peel_current_ref(void);
+
 /*
  * Calls the specified function for each ref file until it returns
  * nonzero, and returns the value.  Please note that it is not safe to
diff --git a/upload-pack.c b/upload-pack.c
index 8f4703b..cdf43b0 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -736,8 +736,9 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 		" include-tag multi_ack_detailed";
 	struct object *o = lookup_unknown_object(sha1);
 	const char *refname_nons = strip_namespace(refname);
+	const unsigned char *peeled = peel_current_ref();
 
-	if (o->type == OBJ_NONE) {
+	if (!peeled && o->type == OBJ_NONE) {
 		o->type = sha1_object_info(sha1, NULL);
 		if (o->type < 0)
 		    die("git upload-pack: cannot find object %s:", sha1_to_hex(sha1));
@@ -756,11 +757,13 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 		o->flags |= OUR_REF;
 		nr_our_refs++;
 	}
-	if (o->type == OBJ_TAG) {
+	if (!peeled && o->type == OBJ_TAG) {
 		o = deref_tag_noverify(o);
 		if (o)
-			packet_write(1, "%s %s^{}\n", sha1_to_hex(o->sha1), refname_nons);
+			peeled = o->sha1;
 	}
+	if (peeled && !is_null_sha1(peeled))
+		packet_write(1, "%s %s^{}\n", sha1_to_hex(peeled), refname_nons);
 	return 0;
 }
Ævar Arnfjörð Bjarmason· Oct 3, 2012, 23:54 UTC · re: Jeff King · lore

Re: upload-pack is slow with lots of refs

On Thu, Oct 4, 2012 at 1:15 AM, Jeff King <peff@peff.net> wrote:
Show 45 quoted lines
> On Thu, Oct 04, 2012 at 12:15:47AM +0200, Ævar Arnfjörð Bjarmason wrote:
>
>> I think he was wrong, I tested this on git.git by first creating a lot
>> of tags:
>>
>>      parallel --eta "git tag -a -m"{}" test-again-{}" ::: $(git rev-list HEAD)
>>
>> Then doing:
>>
>>     git pack-refs --all
>>     git repack -A -d
>>
>> And compiled with -g -O3 I get around 1.55 runs/s of git-upload-pack
>> on 1.7.8 and 2.59/s on the master branch.
>
> Thanks for the update, that's more like what I expected.
>
>> FWIW here are my results on the above pathological git.git
>>
>>     $ uname -r; perf --version; echo 0000 | perf record
>> ./git-upload-pack .>/dev/null; perf report | grep -v ^# | head
>>     3.2.0-2-amd64
>>     perf version 3.2.17
>>     [ perf record: Woken up 1 times to write data ]
>>     [ perf record: Captured and wrote 0.026 MB perf.data (~1131 samples) ]
>>         29.08%  git-upload-pack  libz.so.1.2.7       [.] inflate
>>         17.99%  git-upload-pack  libz.so.1.2.7       [.] 0xaec1
>>          6.21%  git-upload-pack  libc-2.13.so        [.] 0x117503
>>          5.69%  git-upload-pack  libcrypto.so.1.0.0  [.] 0x82c3d
>>          4.87%  git-upload-pack  git-upload-pack     [.] find_pack_entry_one
>>          3.18%  git-upload-pack  ld-2.13.so          [.] 0x886e
>>          2.96%  git-upload-pack  libc-2.13.so        [.] vfprintf
>>          2.83%  git-upload-pack  git-upload-pack     [.] search_for_subdir
>>          1.56%  git-upload-pack  [kernel.kallsyms]   [k] do_raw_spin_lock
>>          1.36%  git-upload-pack  libc-2.13.so        [.] vsnprintf
>>
>> I wonder why your report doesn't note any time in libz. This is on
>> Debian testing, maybe your OS uses different strip settings so it
>> doesn't show up?
>
> Mine was on Debian unstable. The difference is probably that I have 400K
> refs, but only 12K unique ones (this is the master alternates repo
> containing every ref from every fork of rails/rails on GitHub). So I
> spend proportionally more time fiddling with refs and outputting than
> I do actually inflating tag objects.
An updated profile with your patch:
    $ uname -r; perf --version; echo 0000 | perf record
./git-upload-pack .>/dev/null; perf report | grep -v ^# | head
    3.2.0-2-amd64
    perf version 3.2.17
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.015 MB perf.data (~662 samples) ]
        14.45%  git-upload-pack  libc-2.13.so        [.] 0x78140
        12.13%  git-upload-pack  [kernel.kallsyms]   [k] walk_component
        11.01%  git-upload-pack  libc-2.13.so        [.] _IO_getline_info
        10.74%  git-upload-pack  git-upload-pack     [.] find_pack_entry_one
         8.96%  git-upload-pack  [kernel.kallsyms]   [k] __mmdrop
         8.64%  git-upload-pack  git-upload-pack     [.] sha1_to_hex
         6.73%  git-upload-pack  libc-2.13.so        [.] vfprintf
         4.07%  git-upload-pack  libc-2.13.so        [.] strchrnul
         4.00%  git-upload-pack  libc-2.13.so        [.] getenv
         3.37%  git-upload-pack  git-upload-pack     [.] packet_write
> Hmm. It seems like we should not need to open the tags at all. The main
> reason is to produce the "peeled" advertisement just after it. But for a
> packed ref with a modern version of git that supports the "peeled"
> extension, we should already have that information.

B.t.w. do you plan to submit this as a non-hack, I'd like to have it in git.git, so if you're not going to I could pick it up and clean it up a bit. But I think it would be better coming from you.

Ævar Arnfjörð Bjarmason· Oct 3, 2012, 22:32 UTC · re: Jeff King · lore

Re: upload-pack is slow with lots of refs

On Wed, Oct 3, 2012 at 8:03 PM, Jeff King <peff@peff.net> wrote:
Show 20 quoted lines
> What version of git are you using?  In the past year or so, I've made
> several tweaks to speed up large numbers of refs, including:
>
>   - cff38a5 (receive-pack: eliminate duplicate .have refs, v1.7.6); note
>     that this only helps if they are being pulled in by an alternates
>     repo. And even then, it only helps if they are mostly duplicates;
>     distinct ones are still O(n^2).
>
>   - 7db8d53 (fetch-pack: avoid quadratic behavior in remove_duplicates)
>     a0de288 (fetch-pack: avoid quadratic loop in filter_refs)
>     Both in v1.7.11. I think there is still a potential quadratic loop
>     in mark_complete()
>
>   - 90108a2 (upload-pack: avoid parsing tag destinations)
>     926f1dd (upload-pack: avoid parsing objects during ref advertisement)
>     Both in v1.7.10. Note that tag objects are more expensive to
>     advertise than commits, because we have to load and peel them.
>
> Even with those patches, though, I found that it was something like ~2s
> to advertise 100,000 refs.

FWIW I bisected between 1.7.9 and 1.7.10 and found that the point at which it went from 1.5/s to 2.5/s upload-pack runs on the pathological git.git repository was none of those, but:

    ccdc6037fe - parse_object: try internal cache before reading object db
Jeff King· Oct 3, 2012, 23:21 UTC · re: Ævar Arnfjörð Bjarmason · lore

Re: upload-pack is slow with lots of refs

On Thu, Oct 04, 2012 at 12:32:35AM +0200, Ævar Arnfjörð Bjarmason wrote:
Show 27 quoted lines
> On Wed, Oct 3, 2012 at 8:03 PM, Jeff King <peff@peff.net> wrote:
> > What version of git are you using?  In the past year or so, I've made
> > several tweaks to speed up large numbers of refs, including:
> >
> >   - cff38a5 (receive-pack: eliminate duplicate .have refs, v1.7.6); note
> >     that this only helps if they are being pulled in by an alternates
> >     repo. And even then, it only helps if they are mostly duplicates;
> >     distinct ones are still O(n^2).
> >
> >   - 7db8d53 (fetch-pack: avoid quadratic behavior in remove_duplicates)
> >     a0de288 (fetch-pack: avoid quadratic loop in filter_refs)
> >     Both in v1.7.11. I think there is still a potential quadratic loop
> >     in mark_complete()
> >
> >   - 90108a2 (upload-pack: avoid parsing tag destinations)
> >     926f1dd (upload-pack: avoid parsing objects during ref advertisement)
> >     Both in v1.7.10. Note that tag objects are more expensive to
> >     advertise than commits, because we have to load and peel them.
> >
> > Even with those patches, though, I found that it was something like ~2s
> > to advertise 100,000 refs.
> 
> FWIW I bisected between 1.7.9 and 1.7.10 and found that the point at
> which it went from 1.5/s to 2.5/s upload-pack runs on the pathological
> git.git repository was none of those, but:
> 
>     ccdc6037fe - parse_object: try internal cache before reading object db

Ah, yeah, I forgot about that one. That implies that you have a lot of refs pointing to the same objects (since the benefit of that commit is to avoid reading from disk when we have already seen it).

Out of curiosity, what does your repo contain? I saw a lot of speedup with that commit because my repos are big object stores, where we have the same duplicated tag refs for every fork of the repo.

-Peff
Ævar Arnfjörð Bjarmason· Oct 3, 2012, 23:47 UTC · re: Jeff King · lore

Re: upload-pack is slow with lots of refs

On Thu, Oct 4, 2012 at 1:21 AM, Jeff King <peff@peff.net> wrote:
Show 37 quoted lines
> On Thu, Oct 04, 2012 at 12:32:35AM +0200, Ævar Arnfjörð Bjarmason wrote:
>
>> On Wed, Oct 3, 2012 at 8:03 PM, Jeff King <peff@peff.net> wrote:
>> > What version of git are you using?  In the past year or so, I've made
>> > several tweaks to speed up large numbers of refs, including:
>> >
>> >   - cff38a5 (receive-pack: eliminate duplicate .have refs, v1.7.6); note
>> >     that this only helps if they are being pulled in by an alternates
>> >     repo. And even then, it only helps if they are mostly duplicates;
>> >     distinct ones are still O(n^2).
>> >
>> >   - 7db8d53 (fetch-pack: avoid quadratic behavior in remove_duplicates)
>> >     a0de288 (fetch-pack: avoid quadratic loop in filter_refs)
>> >     Both in v1.7.11. I think there is still a potential quadratic loop
>> >     in mark_complete()
>> >
>> >   - 90108a2 (upload-pack: avoid parsing tag destinations)
>> >     926f1dd (upload-pack: avoid parsing objects during ref advertisement)
>> >     Both in v1.7.10. Note that tag objects are more expensive to
>> >     advertise than commits, because we have to load and peel them.
>> >
>> > Even with those patches, though, I found that it was something like ~2s
>> > to advertise 100,000 refs.
>>
>> FWIW I bisected between 1.7.9 and 1.7.10 and found that the point at
>> which it went from 1.5/s to 2.5/s upload-pack runs on the pathological
>> git.git repository was none of those, but:
>>
>>     ccdc6037fe - parse_object: try internal cache before reading object db
>
> Ah, yeah, I forgot about that one. That implies that you have a lot of
> refs pointing to the same objects (since the benefit of that commit is
> to avoid reading from disk when we have already seen it).
>
> Out of curiosity, what does your repo contain? I saw a lot of speedup
> with that commit because my repos are big object stores, where we have
> the same duplicated tag refs for every fork of the repo.

Things are much faster with your monkeypatch, got up to around 10 runs/s.

The repository mainly contains a lot of git-deploy[1] generated tags which are added for every rollout to several subsystems.

Of the ~50k references in the repo 75% point to a commit that no other reference points to. Around 98% of the references are annotated tags, the rest are branches.

1. https://github.com/git-deploy/git-deploy
Junio C Hamano· Oct 3, 2012, 19:13 UTC · re: Ævar Arnfjörð Bjarmason · lore

Re: upload-pack is slow with lots of refs

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:
Show 5 quoted lines
> I'm creating a system where a lot of remotes constantly fetch from a
> central repository for deployment purposes, but I've noticed that even
> with a remote.$name.fetch configuration to only get certain refs a
> "git fetch" will still call git-upload pack which will provide a list
> of all references.

It has been observed that the sender has to advertise megabytes of refs because it has to speak first before knowing what the receiver wants, even when the receiver is interested in getting updates from only one of them, or worse yet, when the receiver is only trying to peek the ref it is interested has been updated.

I do not think upload-pack that runs on the sender side with millions refs, when asked for a single "want", feeds all the refs that it has to the revision machinery, and if you observed it does, I cannot explain why it happens.

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