[PATCH v3 2/2] midx: stop repeatedly looking up nonexistent packfiles

The multi-pack index acts as a cache across a set of packfiles so that we can quickly look up which of those packfiles contains a given object. As such, the multi-pack index naturally needs to be updated every time one of the packfiles goes away, or otherwise the multi-pack index has grown stale.

A stale multi-pack index should be handled gracefully by Git though, and in fact it is: if the indexed pack cannot be found we simply ignore it and eventually we fall back to doing the object lookup by just iterating through all packs, even if those aren't indexed.

But while this fallback works, it has one significant downside: we don't cache the fact that a pack has vanished. This leads to us repeatedly trying to look up the same pack only to realize that it (still) doesn't exist.

This issue can be easily demonstrated by creating a repository with a stale multi-pack index and a couple of objects. We do so by creating a repository with two packfiles, both of which are indexed by the multi-pack index, and then repack those two packfiles. Note that we have to move the multi-pack-index before doing the final repack, as Git knows to delete it otherwise.

    $ git init repo
    $ cd repo/
    $ git config set maintenance.auto false
    $ for i in $(seq 1000); do printf "%d-original" $i >file-$i; done
    $ git add .
    $ git commit -moriginal
    $ git repack -dl
    $ for i in $(seq 1000); do printf "%d-modified" $i >file-$i; done
    $ git commit -a -mmodified
    $ git repack -dl
    $ git multi-pack-index write
    $ mv .git/objects/pack/multi-pack-index .
    $ git repack -Adl
    $ mv multi-pack-index .git/objects/pack/

Commands that cause a lot of objects lookups will now repeatedly invoke `add_packed_git()`, which leads to three failed access(3p) calls as well as one failed stat(3p) call. The following strace for example is done for `git log --patch` in the above repository:

    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
     74.67    0.024693           1     18038     18031 access
     25.33    0.008378           1      6045      6017 newfstatat
    ------ ----------- ----------- --------- --------- ----------------
    100.00    0.033071           1     24083     24048 total

Fix the issue by introducing a negative lookup cache for indexed packs. This cache works by simply storing an invalid pointer for a missing pack when `prepare_midx_pack()` fails to look up the pack. Most users of the `packs` array don't need to be adjusted, either, as they all know to call `prepare_midx_pack()` before accessing the array.

With this change in place we can now see a significantly reduced number of syscalls:

    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
     73.58    0.000323           5        60        28 newfstatat
     26.42    0.000116           5        23        16 access
    ------ ----------- ----------- --------- --------- ----------------
    100.00    0.000439           5        83        44 total

Furthermore, this change also results in a speedup:

    Benchmark 1: git log --patch (revision = HEAD~)
      Time (mean ± σ):      50.4 ms ±   2.5 ms    [User: 22.0 ms, System: 24.4 ms]
      Range (min … max):    45.4 ms …  54.9 ms    53 runs

    Benchmark 2: git log --patch (revision = HEAD)
      Time (mean ± σ):      12.7 ms ±   0.4 ms    [User: 11.1 ms, System: 1.6 ms]
      Range (min … max):    12.4 ms …  15.0 ms    191 runs

    Summary
      git log --patch (revision = HEAD) ran
        3.96 ± 0.22 times faster than git log --patch (revision = HEAD~)

In the end, it should in theory never be necessary to have this negative lookup cache given that we know to update the multi-pack index together with repacks. But as the change is quite contained and as the speedup can be significant as demonstrated above, it does feel sensible to have the negative lookup cache regardless.

Based-on-patch-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 midx.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)