On Mon, Nov 12 2018, Ævar Arnfjörð Bjarmason wrote:
> On Mon, Nov 12 2018, Jeff King wrote:
>
>> On Mon, Nov 12, 2018 at 05:01:02PM +0100, Ævar Arnfjörð Bjarmason wrote:
>>
>>> > There's some obvious hand-waving in the paragraphs above. I would love
>>> > it if somebody with an NFS system could do some before/after timings
>>> > with various numbers of loose objects, to get a sense of where the
>>> > breakeven point is.
>>> >
>>> > My gut is that we do not need the complexity of a cache-size limit, nor
>>> > of a config option to disable this. But it would be nice to have a real
>>> > number where "reasonable" ends and "pathological" begins. :)
>>>
>>> I'm happy to test this on some of the NFS we have locally, and started
>>> out with a plan to write some for-loop using the low-level API (so it
>>> would look up all 256), fake populate .git/objects/?? with N number of
>>> objects etc, but ran out of time.
>>>
>>> Do you have something ready that you think would be representative and I
>>> could just run? If not I'll try to pick this up again...
>>
>> No, but they don't even really need to be actual objects. So I suspect
>> something like:
>>
>> git init
>> for i in $(seq 256); do
>> i=$(printf %02x $i)
>> mkdir -p .git/objects/$i
>> for j in $(seq --format=%038g 1000); do
>> echo foo >.git/objects/$i/$j
>> done
>> done
>> git index-pack -v --stdin </path/to/git.git/objects/pack/XYZ.pack
>>
>> might work (for various values of 1000). The shell loop would probably
>> be faster as perl, too. :)
>>
>> Make sure you clear the object directory between runs, though (otherwise
>> the subsequent index-pack's really do find collisions and spend time
>> accessing the objects).
>>
>> If you want real objects, you could probably just dump a bunch of
>> sequential blobs to fast-import, and then pipe the result to
>> unpack-objects.
>>
>> -Peff
>
> I did a very ad-hoc test against a NetApp filer using the test script
> quoted at the end of this E-Mail. The test compared origin/master, this
> branch of yours, and my core.checkCollisions=false branch.
>
> When run with DBD-mysql.git (just some random ~1k commit repo I had):
>
> $ GIT_PERF_REPEAT_COUNT=3 GIT_PERF_MAKE_OPTS='-j56 CFLAGS="-O3"' ./run origin/master peff/jk/loose-cache avar/check-collisions-config p0008-index-pack.sh
>
> I get:
>
> Test origin/master peff/jk/loose-cache avar/check-collisions-config
> ------------------------------------------------------------------------------------------------------------------------
> 0008.2: index-pack with 256*1 loose objects 4.31(0.55+0.18) 0.41(0.40+0.02) -90.5% 0.23(0.36+0.01) -94.7%
> 0008.3: index-pack with 256*10 loose objects 4.37(0.45+0.21) 0.45(0.40+0.02) -89.7% 0.25(0.38+0.01) -94.3%
> 0008.4: index-pack with 256*100 loose objects 4.47(0.53+0.23) 0.67(0.63+0.02) -85.0% 0.24(0.38+0.01) -94.6%
> 0008.5: index-pack with 256*250 loose objects 5.01(0.67+0.30) 1.04(0.98+0.06) -79.2% 0.24(0.37+0.01) -95.2%
> 0008.6: index-pack with 256*500 loose objects 5.11(0.57+0.21) 1.81(1.70+0.09) -64.6% 0.25(0.38+0.01) -95.1%
> 0008.7: index-pack with 256*750 loose objects 5.12(0.60+0.22) 2.54(2.38+0.14) -50.4% 0.24(0.38+0.01) -95.3%
> 0008.8: index-pack with 256*1000 loose objects 4.52(0.52+0.21) 3.36(3.17+0.17) -25.7% 0.23(0.36+0.01) -94.9%
>
> I then hacked it to test against git.git, but skipped origin/master for
> that one because it takes *ages*. So just mine v.s. yours:
>
> $ GIT_PERF_REPEAT_COUNT=3 GIT_PERF_MAKE_OPTS='-j56 CFLAGS="-O3"' ./run peff/jk/loose-cache avar/check-collisions-config p0008-index-pack.sh
> [...]
> Test peff/jk/loose-cache avar/check-collisions-config
> ---------------------------------------------------------------------------------------------------
> 0008.2: index-pack with 256*1 loose objects 12.57(28.72+0.61) 12.68(29.36+0.62) +0.9%
> 0008.3: index-pack with 256*10 loose objects 12.77(28.75+0.61) 12.50(28.88+0.56) -2.1%
> 0008.4: index-pack with 256*100 loose objects 13.20(29.49+0.66) 12.38(28.58+0.60) -6.2%
> 0008.5: index-pack with 256*250 loose objects 14.10(30.59+0.64) 12.54(28.22+0.57) -11.1%
> 0008.6: index-pack with 256*500 loose objects 14.48(31.06+0.74) 12.43(28.59+0.60) -14.2%
> 0008.7: index-pack with 256*750 loose objects 15.31(31.91+0.74) 12.67(29.23+0.64) -17.2%
> 0008.8: index-pack with 256*1000 loose objects 16.34(32.84+0.76) 13.11(30.19+0.68) -19.8%
>
> So not much of a practical difference perhaps. But then again this isn't
> a very realistic test case of anything. Rarely are you going to push a
> history of something the size of git.git into a repo with this many
> loose objects.
>
> Using sha1collisiondetection.git is I think the most realistic scenario,
> i.e. you'll often end up fetching/pushing something roughly the size of
> its entire history on a big repo, and with it:
>
> Test peff/jk/loose-cache avar/check-collisions-config
> ---------------------------------------------------------------------------------------------------
> 0008.2: index-pack with 256*1 loose objects 0.16(0.04+0.01) 0.05(0.03+0.00) -68.8%
> 0008.3: index-pack with 256*10 loose objects 0.19(0.04+0.02) 0.05(0.02+0.00) -73.7%
> 0008.4: index-pack with 256*100 loose objects 0.32(0.17+0.02) 0.04(0.02+0.00) -87.5%
> 0008.5: index-pack with 256*250 loose objects 0.57(0.41+0.03) 0.04(0.02+0.00) -93.0%
> 0008.6: index-pack with 256*500 loose objects 1.02(0.83+0.06) 0.04(0.03+0.00) -96.1%
> 0008.7: index-pack with 256*750 loose objects 1.47(1.24+0.10) 0.04(0.02+0.00) -97.3%
> 0008.8: index-pack with 256*1000 loose objects 1.94(1.70+0.10) 0.04(0.02+0.00) -97.9%
>
> As noted in previous threads I have an in-house monorepo where (due to
> expiry policies) loose objects hover around the 256*250 mark.
>
> The script, which is hacky as hell and takes shortcuts not to re-create
> the huge fake loose object collection every time (takes ages). Perhaps
> you're interested in incorporating some version of this into a v2. To be
> useful it should take some target path as an env variable.
I forgot perhaps the most useful metric. Testing against origin/master
too on the sha1collisiondetection.git repo, which as noted above I think
is a good stand-in for making a medium sized push to a big repo. This
shows when the loose cache becomes counterproductive:
Test origin/master peff/jk/loose-cache avar/check-collisions-config
-------------------------------------------------------------------------------------------------------------------------
0008.2: index-pack with 256*1 loose objects 0.42(0.04+0.03) 0.17(0.04+0.00) -59.5% 0.04(0.03+0.00) -90.5%
0008.3: index-pack with 256*10 loose objects 0.49(0.04+0.03) 0.19(0.04+0.01) -61.2% 0.04(0.02+0.00) -91.8%
0008.4: index-pack with 256*100 loose objects 0.49(0.04+0.04) 0.33(0.18+0.01) -32.7% 0.05(0.02+0.00) -89.8%
0008.5: index-pack with 256*250 loose objects 0.54(0.03+0.04) 0.59(0.43+0.02) +9.3% 0.04(0.02+0.01) -92.6%
0008.6: index-pack with 256*500 loose objects 0.49(0.04+0.03) 1.04(0.83+0.07) +112.2% 0.04(0.02+0.00) -91.8%
0008.7: index-pack with 256*750 loose objects 0.56(0.04+0.05) 1.50(1.28+0.08) +167.9% 0.04(0.02+0.00) -92.9%
0008.8: index-pack with 256*1000 loose objects 0.54(0.05+0.03) 1.95(1.68+0.13) +261.1% 0.04(0.02+0.00) -92.6%
I still think it's best to take this patch series since it's unlikely
we're making anything worse in practice, the >50k objects case is a
really high number, which I don't think is worth worrying about.
But I am somewhat paranoid about the potential performance
regression. I.e. this is me testing against a really expensive and
relatively well performing NetApp NFS device where the ping stats are:
rtt min/avg/max/mdev = 0.155/0.396/1.387/0.349 ms
So I suspect this might get a lot worse for setups which don't enjoy the
same performance or network locality.
> $ cat t/perf/p0008-index-pack.sh
> #!/bin/sh
>
> test_description="Tests performance of index-pack with loose objects"
>
> . ./perf-lib.sh
>
> test_perf_fresh_repo
>
> test_expect_success 'setup tests' '
> for count in 1 10 100 250 500 750 1000
> do
> if test -d /mnt/ontap_githackers/repo-$count.git
> then
> rm -rf /mnt/ontap_githackers/repo-$count.git/objects/pack
> else
> git init --bare /mnt/ontap_githackers/repo-$count.git &&
> (
> cd /mnt/ontap_githackers/repo-$count.git &&
> for i in $(seq 0 255)
> do
> i=$(printf %02x $i) &&
> mkdir objects/$i &&
> for j in $(seq --format=%038g $count)
> do
> >objects/$i/$j
> done
> done
> )
> fi
> done
> '
>
> for count in 1 10 100 250 500 750 1000
> do
> echo 3 | sudo tee /proc/sys/vm/drop_caches
> test_perf "index-pack with 256*$count loose objects" "
> (
> cd /mnt/ontap_githackers/repo-$count.git &&
> rm -fv objects/pack/*;
> git -c core.checkCollisions=false index-pack -v --stdin </home/aearnfjord/g/DBD-mysql/.git/objects/pack/pack-*.pack
> )
> "
> done
>
> test_done
