Re: EINTR while resizing dsm segment.

On Thu, Apr 2, 2020 at 9:25 PM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
> I provided the subject, and added -hackers.
>
> > Hello,
> > I am running postgres 11.5 and we were having issues with shared segments.
> > So I increased the max_connection as suggested by you guys and reduced my
> > work_mem to 600M.
> >
> > Right now instead, it is the second time I see this error :
> >
> > ERROR: could not resize shared memory segment "/PostgreSQL.2137675995" to
> > 33624064 bytes: Interrupted system call
>
> The function posix_fallocate is protected against EINTR.
>
> | do
> | {
> | rc = posix_fallocate(fd, 0, size);
> | } while (rc == EINTR && !(ProcDiePending || QueryCancelPending));
>
> But not for ftruncate and write. Don't we need to protect them from
> ENTRI as the attached?

We don't handle EINTR for write() generally because that's not
supposed to be necessary on local files (local disks are not "slow
devices", and we document that if you're using something like NFS you
should use its "hard" mount option so that it behaves that way too).
As for ftruncate(), you'd think it'd be similar, and I can't think of
a more local filesystem than tmpfs (where POSIX shmem lives on Linux),
but I can't seem to figure that out from reading man pages; maybe I'm
reading the wrong ones. Perhaps in low memory situations, an I/O wait
path reached by ftruncate() can return EINTR here rather than entering
D state (non-interruptable sleep) or restarting due to our SA_RESTART
flag... anyone know?

Another thought: is there some way for the posix_fallocate() retry
loop to exit because (ProcDiePending || QueryCancelPending), but then
for CHECK_FOR_INTERRUPTS() to do nothing, so that we fall through to
reporting the EINTR?