Eric Wong f043683713 cat-file: use writev(2) if available ...

Using writev here is 20-40% faster than three write syscalls in
succession for smaller (1-10k) objects in the delta base cache.
This advantage decreases as object sizes approach pipe size (64k
on Linux).

writev reduces wakeups and syscalls on the read side as well:
each write(2) syscall may trigger one or more corresponding
read(2) syscalls in the reader.  Attempting atomicity in the
writer via writev also reduces the likelyhood of non-blocking
readers failing with EAGAIN and having to call poll||select
before attempting to read again.

Unfortunately, this turns into a small (1-3%) slowdown for
gigantic objects of a megabyte or more even with after
increasing pipe size to 1MB via the F_SETPIPE_SZ fcntl(2) op.
This slowdown is acceptable to me since the vast majority of
objects are 64K or less for projects I've looked at.

Relying on stdio buffering and fflush(3) after each response was
considered for users without --buffer, but historically cat-file
defaults to being compatible with non-blocking stdout and able
to poll(2) after hitting EAGAIN on write(2).  Using stdio on
files with the O_NONBLOCK flag is (AFAIK) unspecified and likely
subject to portability problems and thus avoided.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2024-08-25 10:54:23 -07:00

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