Horton

Re: Performance degradation on concurrent COPY into a single relation in PG16.

Andres Freund <andres@anarazel.de>

View thread View on postgr.es
From: Andres Freund <andres@anarazel.de>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-07-10T15:44:39Z
Lists: pgsql-hackers 
Hi,

On 2023-07-03 11:59:38 +0900, Masahiko Sawada wrote:
> On Mon, Jul 3, 2023 at 11:55 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > After further investigation, the performance degradation comes from
> > calling posix_fallocate() (called via FileFallocate()) and pwritev()
> > (called via FileZero) alternatively depending on how many blocks we
> > extend by. And it happens only on the xfs filesystem.
>
> FYI, the attached simple C program proves the fact that calling
> alternatively posix_fallocate() and pwrite() causes slow performance
> on posix_fallocate():
>
> $ gcc -o test test.c
> $ time ./test test.1 1
> total   200000
> fallocate       200000
> filewrite       0
>
> real    0m1.305s
> user    0m0.050s
> sys     0m1.255s
>
> $ time ./test test.2 2
> total   200000
> fallocate       100000
> filewrite       100000
>
> real    1m29.222s
> user    0m0.139s
> sys     0m3.139s

On an xfs filesystem, with a very recent kernel:

time /tmp/msw_test /srv/dev/fio/msw 0
total	200000
fallocate	0
filewrite	200000

real	0m0.456s
user	0m0.017s
sys	0m0.439s


time /tmp/msw_test /srv/dev/fio/msw 1
total	200000
fallocate	200000
filewrite	0

real	0m0.141s
user	0m0.010s
sys	0m0.131s


time /tmp/msw_test /srv/dev/fio/msw 2
total	200000
fallocate	100000
filewrite	100000

real	0m0.297s
user	0m0.017s
sys	0m0.280s


So I don't think I can reproduce your problem on that system...

I also tried adding a fdatasync() into the loop, but that just made things
uniformly slow.


I guess I'll try to dig up whether this is a problem in older upstream
kernels, or whether it's been introduced in RHEL.

Greetings,

Andres Freund

Commits

  1. Avoid edge case in pg_visibility test with small shared_buffers

  2. Fix bulk table extension when copying into multiple partitions

  3. hio: Take number of prior relation extensions into account

  4. Fix performance regression in pg_strtointNN_safe functions

  5. Fix performance problem with new COPY DEFAULT code

  6. hio: Use ExtendBufferedRelBy() to extend tables more efficiently

  7. Add VACUUM/ANALYZE BUFFER_USAGE_LIMIT option