Horton

Re: Unportable implementation of background worker start

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, pgsql-hackers@postgresql.org
Date: 2017-04-24T20:16:44Z
Lists: pgsql-hackers 
On 2017-04-21 23:50:41 -0400, Tom Lane wrote:
> I wrote:
> > Attached is a lightly-tested draft patch that converts the postmaster to
> > use a WaitEventSet for waiting in ServerLoop.  I've got mixed emotions
> > about whether this is the direction to proceed, though.
> 
> Attached are a couple of patches that represent a plausible Plan B.
> The first one changes the postmaster to run its signal handlers without
> specifying SA_RESTART.  I've confirmed that that seems to fix the
> select_parallel-test-takes-a-long-time problem on gaur/pademelon.
> The second one uses pselect, if available, to replace the unblock-signals/
> select()/block-signals dance in ServerLoop.  On platforms where pselect
> exists and works properly, that should fix the race condition I described
> previously.  On platforms where it doesn't, we're no worse off than
> before.
> 
> As mentioned in the comments for the second patch, even if we don't
> have working pselect(), the only problem is that ServerLoop's response
> to an interrupt might be delayed by as much as the up-to-1-minute timeout.
> The only existing case where that's really bad is launching multiple
> bgworkers.  I would therefore advocate also changing maybe_start_bgworker
> to start up to N bgworkers per call, where N is large enough to pretty
> much always satisfy simultaneously-arriving requests.  I'd pick 100 or
> so, but am willing to negotiate.
> 
> I think that these patches represent something we could back-patch
> without a lot of trepidation, unlike the WaitEventSet-based approach.
> Therefore, my proposal is to apply and backpatch these changes, and
> call it good for v10.  For v11, we could work on changing the postmaster
> to not do work in signal handlers, as discussed upthread.  That would
> supersede these two patches completely, though I'd still advocate for
> keeping the change in maybe_start_bgworker.
> 
> Note: for testing purposes, these patches are quite independent; just
> ignore the hunk in the second patch that changes a comment added by
> the first one.

Unclear if related, but
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=gharial&dt=2017-04-24%2019%3A30%3A42
has a suspicious timing of failing in a weird way.

- Andres

Commits

  1. Cope with glibc too old to have epoll_create1().

  2. Make latch.c more paranoid about child-process cases.

  3. Allow multiple bgworkers to be launched per postmaster iteration.

  4. Revert "Use pselect(2) not select(2), if available, to wait in postmaster's loop."

  5. Use pselect(2) not select(2), if available, to wait in postmaster's loop.

  6. Run the postmaster's signal handlers without SA_RESTART.

  7. Fix postmaster's handling of fork failure for a bgworker process.

  8. Partially revert commit 536d47bd9d5fce8d91929bee3128fa1d08dbcc57.

  9. Avoid depending on non-POSIX behavior of fcntl(2).

  10. Remove long-obsolete catering for platforms without F_SETFD/FD_CLOEXEC.