sideband: use writev(3p) to send pktlines

sideband: use writev(3p) to send pktlines

Every pktline that we send out via `send_sideband()` currently requires
two syscalls: one to write the pktline's length, and one to send its
data. This typically isn't all that much of a problem, but under extreme
load the syscalls may cause contention in the kernel.

Refactor the code to instead use the newly introduced writev(3p) infra
so that we can send out the data with a single syscall. This reduces the
number of syscalls from around 133,000 calls to write(3p) to around
67,000 calls to writev(3p).

This change leads to a performance improvement for git-upload-pack(1),
but we have to cheat a bit to really make it measurable. Usually, the
time is strongly dominated by generating the packfile itself. But if we
precompute the pack and serve it via the pack-objects hook then we can
essentially eliminate that overhead. The following setup is executed in
the Git repository:

  $ cat >request <<-EOF
  0048want 5ce91c059e41090e7d2cffad39c04af8acf98dc1 side-band no-progress
  00000009done
  EOF
  $ echo 5ce91c059e41090e7d2cffad39c04af8acf98dc1 | git pack-objects --revs --stdout >pack
  $ cat >hook <<-EOF
  #!/bin/sh
  cat >/dev/null
  cat "$(pwd)"/pack
  EOF
  $ chmod u+x hook
  $ git -c uploadpack.packObjectsHook="$(pwd)"/hook upload-pack . <request

Benchmarking the last command leads to the following results:

  Benchmark 1: HEAD~
    Time (mean ± σ):     192.9 ms ±   0.6 ms    [User: 106.5 ms, System: 95.3 ms]
    Range (min … max):   191.7 ms … 194.1 ms    50 runs

  Benchmark 2: HEAD
    Time (mean ± σ):     141.1 ms ±   0.7 ms    [User: 63.2 ms, System: 86.6 ms]
    Range (min … max):   139.8 ms … 142.7 ms    50 runs

  Summary
    HEAD ran
      1.37 ± 0.01 times faster than HEAD~

This might not be impressive in absolute numbers when you also take into
account the time it takes to generate the packfile itself. But GitLab
(and supposedly other forges) have caching mechanisms in place that work
exactly like the above setup, where repeated incoming requests can be
served from the same cached packfile. And in those cases, the impact is
sizeable.

More importantly though, as hinted at above, GitLab has observed in the
past that with enough cache hits we eventually start to saturate a
semaphore in the Linux kernel itself in the pipe write path. This
bottleneck is being moved a bit by having to do less syscalls.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>