Re: Reduced power consumption in autovacuum launcher process

Peter Geoghegan <peter(at)2ndquadrant(dot)com> writes:
> Attached is revision of this patch that now treats the latch in
> PGPROC, waitLatch, as the generic "process latch", rather than just
> using it for sync rep; It is initialised appropriately as a shared
> latch generically, within InitProcGlobal(), and ownership is
> subsequently set within InitProcess(). We were doing so before, though
> only for the benefit of sync rep.

Applied with some corrections, notably:

* Signal handlers mustn't change errno and mustn't assume MyProc is set,
since they might get invoked before it's set or after it's cleared.
Calling SetLatch outside the save/restore of errno is right out, of
course, but I also found that quite a lot of handlers had previously
been hacked to call SetLatch or latch_sigusr1_handler without any
save/restore at all. I'm not sure if any of those were your mistake;
I think a lot of them were pre-existing bugs in the sync rep patch.

* avlauncher loop was missing a ResetLatch call. I also added a comment
explaining the difference from the normal pattern for using WaitLatch.

* I didn't add a SetLatch call in procsignal_sigusr1_handler. I'm
unconvinced that it's necessary, and if it is we probably need to
rethink using SIGUSR1 internally in unix_latch.c. It does not make
sense to set the procLatch when we get a signal that's directed towards
releasing some other latch.

regards, tom lane