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: Rémi Zara <remi_zara@mac.com>, cm@enterprisedb.com, Alvaro Herrera <alvherre@2ndquadrant.com>, pgsql-hackers@postgresql.org
Date: 2017-04-27T00:37:40Z
Lists: pgsql-hackers 
On 2017-04-26 11:42:38 -0400, Tom Lane wrote:
> 1. Let HEAD stand as it is.  We have a problem with slow response to
> bgworker start requests that arrive while ServerLoop is active, but that's
> a pretty tight window usually (although I believe I've seen it hit at
> least once in testing).
> 
> 2. Reinstall the pselect patch, blacklisting NetBSD and HPUX and whatever
> else we find to be flaky.  Then only the blacklisted platforms have the
> problem.

That seems unattractive at this point.  I'm not looking forward to
having to debug more random platforms that implement this badly in a
yet another weird way.


> 3. Go ahead with converting the postmaster to use WaitEventSet, a la
> the draft patch I posted earlier.  I'd be happy to do this if we were
> at the start of a devel cycle, but right now seems a bit late --- not
> to mention that we really need to fix 9.6 as well.

Yea, backpatching this to 9.6 seems like a bigger hammer than
appropriate.  I'm on the fence WRT master, I think there's an argument
to be made that this is going to become a bigger and bigger problem, and
that we'll wish in a year or two that we had fewer releases with
parallelism etc that don't use WaitEventSets.


> I'm leaning to doing #1 plus the maybe_start_bgworker change.  There's
> certainly room for difference of opinion here, though.  Thoughts?

I see you did the bgworker thing - that seems good to me.

Thanks,

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.