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

On 03/07/2023 05:59, Masahiko Sawada wrote:
> On Mon, Jul 3, 2023 at 11:55 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)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

This must be highly dependent on the underlying OS and filesystem. I'm
not seeing that effect on my laptop:

/data$ time /tmp/test test.0 0
total 200000
fallocate 0
filewrite 200000

real 0m1.856s
user 0m0.140s
sys 0m1.688s
/data$ time /tmp/test test.1 1
total 200000
fallocate 200000
filewrite 0

real 0m1.335s
user 0m0.156s
sys 0m1.179s
/data$ time /tmp/test test.2 2
total 200000
fallocate 100000
filewrite 100000

real 0m2.159s
user 0m0.165s
sys 0m1.880s

/data$ uname -a
Linux heikkilaptop 6.0.0-6-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.0.12-1
(2022-12-09) x86_64 GNU/Linux

/data is an nvme drive with ext4 filesystem.

--
Heikki Linnakangas
Neon (https://neon.tech)