[GSOC] How to improve the performance of git cat-file --batch

After reusing ref-filter logic for cat-file --batch, the function of cat-file --batch has been enhanced, but the performance of cat-file is severely degraded. So we need a better solution to solve it. its last version is here: https://lore.kernel.org/git/pull.993.v2.git.1626363626.gitgitgadget@gmail.com/

Use google doc to show some of my recent ideas: https://docs.google.com/document/d/1hyM-ecMd8C_TJ3OsOn46nMr8ZxCUxeK7HKj-bRkq3sk/edit?usp=sharing

Anyone is welcome to comment and suggest better solutions!

Thanks! -- ZheNing Hu

On Sat, Jul 24 2021, ZheNing Hu wrote:

Having looked at that Google Doc it seems to just discuss "Plan A", or there some multi-doc magic and there's a "Plan B", or...? I'm not very familiar with Google Docs.

Anyway, I'd encourage you to just send this as an E-Mail to the mailing list.

Having skimmed it I'm a bit confused about this in reference to performance generally. I haven't looked into the case you're discussing, but as I noted in https://lore.kernel.org/git/87im1p6x34.fsf@evledraar.gmail.com/ the profiling clearly shows that the main problem is that you've added object lookups we skipped before.

I think any plan to improve performance should start with a perf test, see which of your patches have performance regressions, if some don't and are otherwise OK perhaps those can go in earlier.

Then we'll have the smallest possible set of changes that's correct, but introduces performance regressions, and can look at what those are doing to slow things down...

Ævar Arnfjörð Bjarmason <avarab@gmail.com> 于2021年7月25日周日 上午5:23写道：

There may be several plan that have not been written to it for the time being.

Ok, I can migrate them here.

Yeah, you showed me last time that lookup_object() took up a lot of time.

Makes sense. Last time the test performance of commits is here: https://lore.kernel.org/git/CAOLTT8RR4+tUuT2yc2PDL9NwCburW8bM_Sht6nhKJ_fYV8fGsQ@mail.gmail.com/

The main commit with performance degradation is "[GSOC] cat-file: reuse ref-filter logic", we need to focus on it.

Ok.

Thanks. -- ZheNing Hu

On Sun, Jul 25, 2021 at 2:04 PM ZheNing Hu <adlternative@gmail.com> wrote:

Could the document explain with some details why there are more calls to lookup_object()? For example it could take an example `git cat-file --batch ...` command (if possible a simple one), and say which functions like lookup_object() it was using (and how many times) to get the data it needs before using the ref-filter logic, and then the same information after using the ref-filter logic.

It could be nice if there were also some data about how much time used to be spent in lookup_object() and how much time is now spent there, and how this compares with the whole slowdown we are seeing. If Ævar already showed that, you can of course reuse what he already did.

The GIT_TRACE_PERFORMANCE, GIT_TRACE2_PERF env variables and the associated trace_*() and trace2_*() functions might help you with measuring how much time is spent in different functions.

Christian Couder <christian.couder@gmail.com> 于2021年7月26日周一 下午5:38写道：

Sorry but this time I use gprof but can’t observe the same effect as before. lookup_object() is indeed a part of the time overhead, but its proportion is not very large this time.

This is my test for git cat-file --batch --batch-all-objects >/dev/null:

daab8a564: The fifth batch (upstream/master)

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 38.13      0.61     0.61  1968866     0.00     0.00  patch_delta
 13.75      0.83     0.22  6568488     0.00     0.00
unpack_object_header_buffer
 11.25      1.01     0.18   344036     0.00     0.00  unpack_entry
  7.50      1.13     0.12  1964667     0.00     0.00  hashmap_remove
  6.88      1.24     0.11  6153182     0.00     0.00  hashmap_get
  1.88      1.27     0.03  7746299     0.00     0.00  zlib_post_call
  1.88      1.30     0.03   842731     0.00     0.00  bsearch_hash
  1.88      1.33     0.03   827663     0.00     0.00  nth_packed_object_offset
  1.25      1.35     0.02 15385422     0.00     0.00  use_pack
  1.25      1.37     0.02  6236120     0.00     0.00  get_delta_base
  1.25      1.39     0.02  2581859     0.00     0.00  git_inflate_end
  1.25      1.41     0.02   826650     0.00     0.00
do_oid_object_info_extended
  1.25      1.43     0.02   826650     0.00     0.00  find_pack_entry
  1.25      1.45     0.02   825692     0.00     0.00  packed_to_object_type
  1.25      1.47     0.02   378521     0.00     0.00  get_size_from_delta

d3b5272a94: [GSOC] cat-file: reuse ref-filter logic

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 27.06      0.59     0.59  1968866     0.00     0.00  patch_delta
 16.51      0.95     0.36  2202293     0.00     0.00
unpack_object_header_buffer
 13.76      1.25     0.30  5327015     0.00     0.00  hashmap_get
 11.47      1.50     0.25   344036     0.00     0.00  unpack_entry
  8.72      1.69     0.19   521278     0.00     0.00  lookup_object
  4.13      1.78     0.09  1964667     0.00     0.00  hashmap_remove
  2.75      1.84     0.06   348709     0.00     0.00  get_object
  2.29      1.89     0.05        1     0.05     2.17  oid_array_for_each_unique
  1.38      1.92     0.03  6373452     0.00     0.00  use_pack
  0.92      1.94     0.02  2202293     0.00     0.00  unpack_compressed_entry
  0.92      1.96     0.02  1394836     0.00     0.00  grab_sub_body_contents
  0.92      1.98     0.02   348709     0.00     0.00  create_object
  0.92      2.00     0.02   348709     0.00     0.00  format_ref_array_item
  0.92      2.02     0.02    74557     0.00     0.00  fill_commit_graph_info

Because we called parse_object_buffer() in get_object(), lookup_object() is called indirectly...

We can see that some functions are called the same times: patch_delta(), unpack_entry(), hashmap_remove()... But after using my patch, format_ref_array_item(), grab_sub_body_contents(), get_object(), lookup_object() begin to occupy a certain proportion.

This is my test for git cat-file --batch-check --batch-all-objects >/dev/null:

daab8a564: The fifth batch (upstream/master)

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 47.83      0.11     0.11  3385670     0.00     0.00
unpack_object_header_buffer
 13.04      0.14     0.03  6941590     0.00     0.00  use_pack
  8.70      0.16     0.02  1046130     0.00     0.00  expand_format
  4.35      0.17     0.01   349013     0.00     0.00  oid_array_append
  4.35      0.18     0.01   348710     0.00     0.00  strbuf_expand
  4.35      0.19     0.01   348709     0.00     0.00  find_pack_entry
  4.35      0.20     0.01   348230     0.00     0.00  packed_to_object_type
  4.35      0.21     0.01   259719     0.00     0.00  git_inflate
  4.35      0.22     0.01        1    10.00   210.00  oid_array_for_each_unique
  4.35      0.23     0.01                             pack_basename

d3b5272a94: [GSOC] cat-file: reuse ref-filter logic

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 52.00      0.13     0.13  3385670     0.00     0.00
unpack_object_header_buffer
 16.00      0.17     0.04  6941590     0.00     0.00  use_pack
  8.00      0.19     0.02  3296680     0.00     0.00  get_delta_base
  4.00      0.20     0.01   348709     0.00     0.00  find_pack_entry
  4.00      0.21     0.01   348709     0.00     0.00  oid_to_hex
  4.00      0.22     0.01   348709     0.00     0.00  populate_value
  4.00      0.23     0.01   348230     0.00     0.00  packed_object_info
  4.00      0.24     0.01   348230     0.00     0.00  packed_to_object_type
  4.00      0.25     0.01                             void_hashcmp

Similarly, unpack_object_header_buffer(), use_pack(), find_pack_entry(), packed_to_object_type(), they are still called same times as before; populate_value(), packed_object_info(), oid_to_hex() are new.

Thanks. -- ZheNing Hu

On Tue, Jul 27, 2021 at 3:37 AM ZheNing Hu <adlternative@gmail.com> wrote:

Please note that here we are looking for the number of times the lookup_object() function is called. This means that to measure that properly, it might actually be better to have some way to count this number of times the lookup_object() function is called, rather than count the time spent in the function.

For example you could add a trace_printf(...) call in the lookup_object() function, set GIT_TRACE=/tmp/git_trace.log, and then just run `git cat-file --batch ...` and count the number of times the new trace from lookup_object() appears in the log file.

I am not sure gprof is a good tool for this. It looks like it tries to attribute spent times to functions by splitting time between many low level functions, and it doesn't seem like the right approach to me. For example if lookup_object() is called 5% more often, it could be that the excess time is attributed to some low level functions and not to lookup_object() itself.

That's why we might get a more accurate view of what happens by just counting the number of time the function is called.

Now I regret having wrote the above, sorry, as it might not be the best way to look at this.

[...]

It would be nice if you could add a bit more details about how lookup_object() is called (both before and after the changes that degrade performance).

When you say "the same times" I guess you mean that the same amount of time is spent in these functions.

Thanks!

Christian Couder <christian.couder@gmail.com> 于2021年7月28日周三 下午3:34写道：

Ok, therefore we need an accurate number of call times about lookup_object(), although the conclusion is obvious: 0 (upstream/master) and a big number (with my patch).

After adding a trace_printf() in lookup_object(), here is the result:

Checkout to d3b5272a94 [GSOC] cat-file: reuse ref-filter logic

$ GIT_TRACE=/tmp/git_trace.log ggg cat-file --batch --batch-all-objects $ cat /tmp/git_trace.log | wc -l 522710

Checkout to eb27b338a3e7 (upstream/master)

$ rm /tmp/git_trace.log $ GIT_TRACE=/tmp/git_trace.log ggg cat-file --batch --batch-all-objects $ cat /tmp/git_trace.log | wc -l 1

This is the only 1 time left is printed by git.c, which show that after using my patch, we additionally call lookup_object() when we use --batch option. According to the results of the previous gprof test: lookup_object() occupies 8.72% of the total time. (Though below you seem to think that the effect of gprof is not reliable enough.) This may be a place worthy of optimization.

Maybe you are talking about "cumulative seconds" part of gprof? it's "self seconds" part is the number of seconds accounted for by this function alone.

According to https://sourceware.org/binutils/docs/gprof/Flat-Profile.html :

calls This is the total number of times the function was called. If the function was never called, or the number of times it was called cannot be determined (probably because the function was not compiled with profiling enabled), the calls field is blank.

Therefore, there are accurate numbers in the previous gprof test results.

Yes, because his previous results are based on a version of my patch set, and some changes have taken place with my patch later.

After we letting git cat-file --batch reuse the logic of ref-filter, we will use get_object() to grab the object's data. Since we used atom %(raw), it will require us to grab the raw data of the object, oi->info.contentp will be set, parse_object_buffer() in get_object() will be called, parse_object_buffer() calls lookup_commit(), lookup_blob(), lookup_tree(), and lookup_tag(), they call lookup_object(). As we have seen, lookup_object() seems to take a lot of time.

So let us think, can we skip this parse_object_buffer() in some scenarios? parse_object_buffer() parses the data of the object into a "struct object *obj", and then we use this obj feed to grab_values(), and then grab_values() feed obj to grab_tag_values() or grab_commit_values() to handle some logic about %(tag), %(type), %(object), %(tree), %(parent), %(numparent).

But git cat-file --batch can avaid handle there atoms with default format.

Therefore, maybe we can skip parsing object buffer if we really don't care about these atoms.

No... What I want to express is that the number of calls to these functions is same, see gprof "calls part" with "patch_delta()", both are 1968866.

Thanks for hint!!! -- ZheNing Hu

On Wed, Jul 28, 2021 at 3:38 PM ZheNing Hu <adlternative@gmail.com> wrote:

[...]

First, yeah, I hadn't seen the "calls" columns in your gprof reports. Sorry! It's nice to see that your manual check with a trace_printf() function gives the same result as gprof about this though.

Anyway if you agree that it might be a place worthy of optimization, then it might be a good idea to explain the reason for the numerous lookup_object() calls when using the ref-filter code.

[...]

Not sure why you are talking about %(raw). Is the root of the issue that we now use atom %(raw), or how we implemented it?

Yeah, maybe oi->info.contentp should be set only if the user specified one of the atoms that really needs the content provided by parse_object_buffer().

Thanks!