Partitioned packs

I've been running some experiments, as hinted earlier by the discussion about just how much git-index-pack sucks (which, really, isn't much since the gaping memleak is gone now).

These experiments include trying to see if there's a noticeable performance improvement by splitting out objects of different types into different packs. So far, it definitely seems to make a difference, though not the one I was initially expecting. For all of these tests, I did 'sysctl -w vm.drop_caches=3' before running, to effectively simulate a cold-cache run.

Single 3.1GB pack file containing all commits, blobs, and trees First run (cold cache): git-rev-list --all > /dev/null 5.52s user 0.32s system 45% cpu 12.872 total git-blame -- kdelibs/kdeui/kmenubar.cpp 0.00s user 0.01s system 0% cpu 40.218s total git-archive --format=tar HEAD -- kdelibs >> /dev/null 0.48s user 0.10s system 5% cpu 10.143 total

Subsequent runs (warm cache): git-rev-list --all > /dev/null 5.19s user 0.48s system 98% cpu 5.750 total git-blame -- kdelibs/kdeui/kmenubar.cpp 0.00s user 0.00s system 0% cpu 11.960 total git-archive --format=tar HEAD -- kdelibs >> /dev/null 0.43s user 0.04s system 100% cpu 0.472 total

Single pack for commit objects and another pack for the rest First run (cold cache): git-rev-list --all > /dev/null 5.84s user 0.34s system 31% cpu 19.427 total git-blame -- kdelibs/kdeui/kmenubar.cpp 0.00s user 0.00s system 0% cpu 9:42.74 total git-archive --format=tar HEAD -- kdelibs >> /dev/null 0.50s user 0.26s system 0% cpu 1:35.44 total

Subsequent runs (warm cache): git-rev-list --all > /dev/null 5.94s user 0.26s system 99% cpu 6.204 total git-blame -- kdelibs/kdeui/kmenubar.cpp 0.00s user 0.00s system 0% cpu 12.394 total git-archive --format=tar HEAD -- kdelibs >> /dev/null 0.41s user 0.07s system 98% cpu 0.486 total

Fully-partitioned separate packs for commit, tree, and blob objects First run (cold cache): git-rev-list --all > /dev/null 6.24s user 0.32s system 25% cpu 25.689 total git-blame -- kdelibs/kdeui/kmenubar.cpp 0.00s user 0.00s system 0% cpu 1:08.76 total git-archive --format=tar HEAD -- kdelibs >> /dev/null 0.38s user 0.30s system 0% cpu 1:35.89 total

Subsequent runs (warm cache): git-rev-list --all > /dev/null 6.28s user 0.24s system 99% cpu 6.527 total git-blame -- kdelibs/kdeui/kmenubar.cpp 0.00s user 0.00s system 0% cpu 13.895 total git-archive --format=tar HEAD -- kdelibs >> /dev/null 0.42s user 0.06s system 99% cpu 0.476 total

I packed all of these using --delta-base-offset, with a window of 100 and a depth of 10.

-clee

On Tue, 3 Apr 2007, Chris Lee wrote:

I wonder what order the packs ended up in. if git had to go through the wrong pack completely first before finding the pack that it needed, that coudl account for extra time.

is it worth makeing up single packs that order the three different types of object differently within the one pack to see what difference it makes to have to walk past all the blobs to get to the commits and trees?

David Lang

"Chris Lee" <clee@kde.org> writes:

Are you running on a 64-bit machine or 32-bit?

I wonder what the numbers would be if you partition into the same number of packs of similar sizes as your experiment, but partitioning based on not by type but by age or other factors.

What I am getting at is that you may not be seeing the effect of access pattern based on the type at all. For example, the performance can be affected by other factors, such as necessity to use smaller number of pack_windows per pack. use_pack() iterates through the currently active windows on a linked list per pack, and a window is 32MB on 32-bit machines, so you would literally need hundreds of them to access that 3GB pack (the total is limited to 256MB so 8 windows are recycled). It is possible that simply using more packs and knowing which pack you need to access upfront may be cutting down the cost of finding the pack window to use. A single pack would have a linked list of 8 active windows, while two packs would have one linked list of each, so the average linear search cost would be half.

On Tue, 3 Apr 2007, Chris Lee wrote:

Ok, the wordwrap makes it a bit hard to read, but it looks like the single-pack always wins. Sometimes by a huge amount.

The reason is simple: not only are single packs well sorted anyway (so if you only look at commits, it will only look at the head of the pack anyway), but a single pack is much faster to look things up in: you can do a single binary lookup.

If you have multiple packs, you *may* be able to do a single binary lookup, but quite often you'll do one *failing* binary lookup, and then go on to the next pack - in other words, you'll do a linear search over a set of binary lookups.

So trying to partition things doesn't help (because the objects are already well sorted), and it does hurt.

That said, for most operations it's probably in the noise. Something bad happened for your "git-blame" thing for the "commits" and "everything else" case. Perhaps just unlucky ordering of packs.

		Linus

On Tue, 3 Apr 2007, Linus Torvalds wrote:

Side note: I think that there *are* cases where partitioned packs can do better, but I think that in order to do better you should

 - partition by "recency", ie put objects that are not reachable from any 
   recent point in older packs.

 - make sure that the "packed_git" list is always sorted so that the older 
   data packs are at the end.

and that should actually speed up many loads, just because the recent objects are all in one pack, and because it's smaller, that pack can be looked up a bit faster.

On the other hand, the power of a log(n) function like a binary search is that lookup in a big pack that is four times the size of four smaller packs is really not all that much more expensive, so the advantage is probably pretty small.

And for things that need old objects (and "git blame" does obviously very much tend to fall into that category), any partitioning is likely to be bad.

So I think partitioning is valid, but my suspicion is that you'd want to partition for *other* reasons than highest performance. Better reasons to have multiple packs:

 - just because you haven't repacked ;)
 - to keep "git repack" times down by marking old big packs as "keep" once 
   they get big enough (the space advantage of packing eventually flattens 
   out, so there's no real overwhelming reason to repack old stuff if you 
   have "enough")
 - filesystem and pack-file limitations (ie the 2**31 limit)

but I doubt performance is ever going to be a really compelling one.

You can obviously always optimize for some very *particular* load by packing optimally for just that one (keep exactly the objects you need in one particular pack, don't even touch any other packs), but I don't think any load is *so* special that you shouldn't think of other loads.

			Linus