[PATCH v3 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 v3:
  - Fix handling of `iov_len` overflows in writev(3p) wrapper.
  - Add another patch that causes us to flush out data instead of
    sending a 0005 keepalive packet.
  - Link to v2: https://lore.kernel.org/r/20260303-pks-upload-pack-write-contention-v2-0-7321830f08fe@pks.im

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: prefer flushing data over sending keepalive
      upload-pack: reduce lock contention when writing packfile data
      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        | 44 +++++++++++++++++++++++++++
 config.mak.uname       |  2 ++
 csum-file.c            | 28 +++++------------
 csum-file.h            | 16 ++++++++--
 meson.build            |  1 +
 sideband.c             | 14 +++++++--
 upload-pack.c          | 81 +++++++++++++++++++++++++++++++++++++++-----------
 wrapper.c              | 41 +++++++++++++++++++++++++
 wrapper.h              |  9 ++++++
 write-or-die.c         |  8 +++++
 write-or-die.h         |  1 +
 14 files changed, 239 insertions(+), 47 deletions(-)

Range-diff versus v2:

 1:  ef3244ffcc =  1:  540b577540 upload-pack: fix debug statement when flushing packfile data
 2:  3297edd609 =  2:  253d119b10 upload-pack: adapt keepalives based on buffering
 -:  ---------- >  3:  960c650063 upload-pack: prefer flushing data over sending keepalive
 3:  84671fb222 =  4:  0e147c41cf upload-pack: reduce lock contention when writing packfile data
 4:  bd1f070cb4 <  -:  ---------- git-compat-util: introduce `cast_size_t_to_ssize_t()`
 5:  d5acbd1584 !  5:  b6fbd89b8e compat/posix: introduce writev(3p) wrapper
    @@ compat/writev.c (new)
     +ssize_t git_writev(int fd, const struct iovec *iov, int iovcnt)
     +{
     +	size_t total_written = 0;
    ++	size_t sum = 0;
    ++
    ++	/*
    ++	 * According to writev(3p), the syscall shall error with EINVAL in case
    ++	 * the sum of `iov_len` overflows `ssize_t`.
    ++	 */
    ++	 for (int i = 0; i < iovcnt; i++) {
    ++		if (iov[i].iov_len > maximum_signed_value_of_type(ssize_t) ||
    ++		    iov[i].iov_len + sum > maximum_signed_value_of_type(ssize_t)) {
    ++			errno = EINVAL;
    ++			return -1;
    ++		}
    ++
    ++		sum += iov[i].iov_len;
    ++	}
     +
     +	for (int i = 0; i < iovcnt; i++) {
     +		const char *bytes = iov[i].iov_base;
    @@ compat/writev.c (new)
     +	}
     +
     +out:
    -+	return cast_size_t_to_ssize_t(total_written);
    ++	return (ssize_t) total_written;
     +}
     
      ## config.mak.uname ##
 6:  e3525cd25e =  6:  0a90ab2f62 wrapper: introduce writev(3p) wrappers
 7:  fe07f2d331 =  7:  5e29cfbf10 sideband: use writev(3p) to send pktlines
 8:  4563ad923f =  8:  32b59d5f29 csum-file: introduce `hashfd_ext()`
 9:  549b3dbefc =  9:  a5b0eb3627 csum-file: drop `hashfd_throughput()`
10:  155f79b128 = 10:  a82959772e builtin/pack-objects: reduce lock contention when writing packfile data

--- base-commit: fb1b83bcddff60463f6e86bb021784c88d0b748c change-id: 20260227-pks-upload-pack-write-contention-435ce01f5fe9