Re: [PATCH] pack-objects: fix performance issues on packing large deltas

From: Duy Nguyen <pclouds@gmail.com>
Date: Jul 21, 2018, 04:07 UTC
Message-ID: <CACsJy8Ck7U3m8khFdiKDzYWKk1ZcKyi32RkQ=a=oFQYHd5bruQ@mail.gmail.com>
In-Reply-To: <CABPp-BF5O2-DQMSjN67HYsHHYHP_VH-N-C=k796NwPTvtwf7gQ@mail.gmail.com>

On Sat, Jul 21, 2018 at 1:52 AM Elijah Newren <newren@gmail.com> wrote:

Show 69 quoted lines

>
> On Fri, Jul 20, 2018 at 10:43 AM, Elijah Newren <newren@gmail.com> wrote:
> > On Fri, Jul 20, 2018 at 8:39 AM, Nguyễn Thái Ngọc Duy <pclouds@gmail.com> wrote:
>
> >> This patch provides a better fallback that is
> >>
> >> - cheaper in terms of cpu and io because we won't have to read
> >>   existing pack files as much
> >>
> >> - better in terms of pack size because the pack heuristics is back to
> >>   2.17.0 time, we do not drop large deltas at all
> >>
> >> If we encounter any delta (on-disk or created during try_delta phase)
> >> that is larger than the 2MB limit, we stop using delta_size_ field for
> >> this because it can't contain such size anyway. A new array of delta
> >> size is dynamically allocated and can hold all the deltas that 2.17.0
> >> can [1]. All current delta sizes are migrated over.
> >>
> >> With this, we do not have to drop deltas in try_delta() anymore. Of
> >> course the downside is we use slightly more memory, even compared to
> >> 2.17.0. But since this is considered an uncommon case, a bit more
> >> memory consumption should not be a problem.
>
> Is the increased memory only supposed to affect the uncommon case?  I
> was able to verify that 2.18.0 used less memory than 2.17.0 (for
> either my repo or linux.git), but I don't see nearly the same memory
> reduction relative to 2.17.0 for linux.git with your new patches.
>
> >> ---
> >>  I'm optimistic that the slowness on linux repo is lock contention
> >>  (because if it's not then I have no clue what is). So let's start
> >>  doing proper patches.
> >
> > I'll be testing this shortly...
>
> Using these definitions for versions:
>
>   fix-v5: https://public-inbox.org/git/20180720052829.GA3852@sigill.intra.peff.net/
> (plus what it depends on)
>   fix-v6: The patch I'm responding to
>   fix-v2: https://public-inbox.org/git/20180718225110.17639-3-newren@gmail.com/
>
> and inspired by
> https://public-inbox.org/git/87po42cwql.fsf@evledraar.gmail.com/, I
> ran
>
>    /usr/bin/time -f 'MaxRSS:%M Time:%e' git gc --aggressive
>
> three times on a beefy box (40 cores, 160GB ram, otherwise quiet
> system).  If I take the lowest MaxRSS, and the lowest time reported
> (regardless of whether from the same run), then the results are as
> follows for linux.git (all cases repacked down to 1279 MB, so removing
> that from the table):
>
> Version  MaxRSS(kB)  Time (s)
> -------  ----------  --------
>  2.17.0   11413236    621.36
>  2.18.0   10987152    621.80
>  fix-v5   11105564    836.29
>  fix-v6   11357304    831.73
>  fix-v2   10873348    619.96
>
> The runtime could swing up to 10 seconds between runs.  The memory use
> also had some variability, but all runs of fix-v2 had lower MaxRSS
> than any runs of 2.18.0 (which surprised me); all runs of 2.18.0 used
> less memory than any run of fix-v6, and all runs of fix-v6 used less
> memory than any run of 2.17.0.  fix-v5 had the most variabiilty in
> MaxRSS, ranging from as low as some 2.18.0 runs up to higher than any
> 2.17.0 runs.

As Peff said, RSS is not a very good way to measure memory usage. valgrind --tool=massif would be the best, or just grep "heap" in /proc/$PID/smaps. fix-v2 should increase struct object_entry's size and memory usage for all repos, while v6 has the same memory usage as 2.17.0 in heap, and extra once it hits a big delta.

Show 8 quoted lines

> ...but maybe that'd change with more than 3 runs of each?
>
> Anyway, this is a lot better than the 1193.67 seconds I saw with
> fix-v4 (https://public-inbox.org/git/20180719182442.GA5796@duynguyen.home/,
> which Peff fixed up into fix-v5), suggesting it is lock contention,
> but we're still about 33% slower than 2.17.0 and we've lost almost all
> of the memory savings.  Somewhat surprisingly, the highly simplistic
> fix-v2 does a lot better on both measures.

Maybe we should avoid locking when the delta is small enough then and see how it goes. This is linux.git so there's no big delta and we would end up not locking at all.

If you do "git repack -adf --threads=8", does the time difference between v6 and 2.17.0 reduce (suggesting we still hit lock contention hard)?

> However, I'm just concentrating on a beefy machine; it may be that v6
> drastically outperforms v2 on weaker hardware?  Can others measure a
> lower memory usage for v6 than v2?

I'll try it with massif on linux.git, but this is a very weak laptop (4GB) so it'll take a while....

Show 24 quoted lines

> Also, for the original repo with the huge deltas that started it all,
> the numbers are (only run once since it takes so long):
>
> Version  Pack (MB)  MaxRSS(kB)  Time (s)
> -------  ---------  ----------  --------
>  2.17.0     5498     43513628    2494.85
>  2.18.0    10531     40449596    4168.94
>  fix-v5     5500     42805716    2577.50
>  fiv-v6     5509     42814836    2605.36
>  fix-v2     5509     41644104    2468.25
>
>
> >>  If we want a quick fix for 2.18.1. I suggest bumping up
> >>  OE_DELTA_SIZE_BITS like Elijah did in his second option. I don't
> >>  think it's safe to fast track this patch.
> >
> > Okay, I'll submit that with a proper commit message then.  Since you
> > also bump OE_DELTA_SIZE_BITS in your patch (though to a different
> > value), it'll require a conflict resolution when merging maint into
> > master, but at least the change won't silently leak through.
>
> ...except now I'm wondering if the commit message should mention that
> it's just a stopgap fix for 2.18.1, or whether it's actually the fix
> that we just want to use going forward as well.

no fix-v2 is really a stopgap. With 2.17.0 (and fix-v6) we pay 80 bytes per object, or 6.5M * 80 = 520 MB heap. fix-v2 increases this struct to 88 bytes so we pay 52 MB extra.

-- 
Duy