Horton

Re: Unportable implementation of background worker start

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgreSQL.org
Date: 2017-04-19T22:24:15Z
Lists: pgsql-hackers 
Tom Lane wrote:

> While I haven't yet tested it, it seems like a fix might be as simple
> as deleting these lines in maybe_start_bgworker:
> 
>             /*
>              * Have ServerLoop call us again.  Note that there might not
>              * actually *be* another runnable worker, but we don't care all
>              * that much; we will find out the next time we run.
>              */
>             StartWorkerNeeded = true;
>             return;
> 
> So I'm wondering what the design rationale was for only starting one
> bgworker per invocation.

The rationale was that there may be other tasks waiting for postmaster
attention, and if there are many bgworkers needing to be started, the
other work may be delayed for a long time.  This is not the first time
that this rationale has been challenged, but so far there hasn't been
any good reason to change it.  One option is to just remove it as you
propose, but a different one is to stop using select(2) in ServerLoop,
because those behavior differences seem to make it rather unusable.

> It also appears to me that do_start_bgworker's treatment of fork
> failure is completely brain dead.  Did anyone really think about
> that case?

Hmm, I probably modelled it on autovacuum without giving that case much
additional consideration.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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.