[PATCH v2 00/10] upload-pack: reduce lock contention when writing packfile data

Hi,

this small patch series fixes some heavy lock contention when writing data from git-upload-pack(1) into pipes. This lock contention can be observed when having hundreds of git-upload-pack(1) processes active at the same time that write data into pipes at dozens of gigabits per second.

I have uploaded the flame graph that clearly shows the lock contention at [1].

Changes in v2:
  - Change the buffer size in git-pack-objects(1) to also reduce the
    number of write syscalls over there.
  - Introduce writev to half the number of syscalls when writing
    pktlines.
  - Use `sizeof(os->buffer)` instead of open-coding its size.
  - Improve keepalive logic in git-upload-pack(1) to account for
    buffering.
  - Link to v1: https://lore.kernel.org/r/20260227-pks-upload-pack-write-contention-v1-0-7166fe255704@pks.im

Thanks!

Patrick

[1]: https://gitlab.com/gitlab-org/git/-/work_items/675

---
Patrick Steinhardt (10):
      upload-pack: fix debug statement when flushing packfile data
      upload-pack: adapt keepalives based on buffering
      upload-pack: reduce lock contention when writing packfile data
      git-compat-util: introduce `cast_size_t_to_ssize_t()`
      compat/posix: introduce writev(3p) wrapper
      wrapper: introduce writev(3p) wrappers
      sideband: use writev(3p) to send pktlines
      csum-file: introduce `hashfd_ext()`
      csum-file: drop `hashfd_throughput()`
      builtin/pack-objects: reduce lock contention when writing packfile data

 Makefile               |  4 ++++
 builtin/pack-objects.c | 23 +++++++++++++++----
 compat/posix.h         | 14 ++++++++++++
 compat/writev.c        | 29 ++++++++++++++++++++++++
 config.mak.uname       |  2 ++
 csum-file.c            | 28 +++++++----------------
 csum-file.h            | 16 ++++++++++++--
 git-compat-util.h      |  8 +++++++
 meson.build            |  1 +
 sideband.c             | 14 +++++++++---
 upload-pack.c          | 60 ++++++++++++++++++++++++++++++++++++++++----------
 wrapper.c              | 41 ++++++++++++++++++++++++++++++++++
 wrapper.h              |  9 ++++++++
 write-or-die.c         |  8 +++++++
 write-or-die.h         |  1 +
 15 files changed, 217 insertions(+), 41 deletions(-)

Range-diff versus v1:

 1:  368a984880 =  1:  81c00e3b52 upload-pack: fix debug statement when flushing packfile data
 -:  ---------- >  2:  01c86a9573 upload-pack: adapt keepalives based on buffering
 2:  f014e8005a !  3:  16bc5661d4 upload-pack: reduce lock contention when writing packfile data
    @@ Commit message
         write(3p) syscalls from 130,000 to 100,000. So overall this doesn't seem
         worth it.
     
    +    Note that the issue could also be fixed by adapting the write buffer
    +    that we use in the downstream git-pack-objects(1) command, and such a
    +    change would have roughly the same result. But the command that
    +    generates the packfile data may not always be git-pack-objects(1) as it
    +    can be changed via "uploadpack.packObjectsHook", so such a fix would
    +    only help in _some_ cases. Regardless of that, we'll also adapt the
    +    write buffer size of git-pack-objects(1) in a subsequent commit.
    +
         Helped-by: Matt Smiley <msmiley@gitlab.com>
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
     
    @@ upload-pack.c: static int relay_pack_data(int pack_objects_out, struct output_st
     +	 * Make sure that we buffer some data before sending it to the client.
     +	 * This significantly reduces the number of write(3p) syscalls.
     +	 */
    -+	if (readsz && os->used < (LARGE_PACKET_DATA_MAX * 2 / 3))
    ++	if (readsz && os->used < (sizeof(os->buffer) * 2 / 3))
     +		return readsz;
     +
      	if (os->used > 1) {
 -:  ---------- >  4:  f42a8a0558 git-compat-util: introduce `cast_size_t_to_ssize_t()`
 -:  ---------- >  5:  830c72f588 compat/posix: introduce writev(3p) wrapper
 -:  ---------- >  6:  733ef129f1 wrapper: introduce writev(3p) wrappers
 -:  ---------- >  7:  18f7429e14 sideband: use writev(3p) to send pktlines
 -:  ---------- >  8:  8eb2aee74d csum-file: introduce `hashfd_ext()`
 -:  ---------- >  9:  a634506b81 csum-file: drop `hashfd_throughput()`
 -:  ---------- > 10:  ca6d52b012 builtin/pack-objects: reduce lock contention when writing packfile data

--- base-commit: 7b2bccb0d58d4f24705bf985de1f4612e4cf06e5 change-id: 20260227-pks-upload-pack-write-contention-435ce01f5fe9