On Tue, Apr 10, 2007 at 02:46:56PM -0400, Tom Lane wrote:
> Kris Kennaway <kris@obsecurity.org> writes:
> >>> Make SYSV semaphores less dumb about process wakeups. Currently
> >>> whenever the semaphore state changes, all processes sleeping on the
> >>> semaphore are woken, even if we only have released enough resources
> >>> for one waiting process to claim.
>
> >> Correct. The behavior Kris describes is surely bad, but it's not
> >> relevant to Postgres' usage of SysV semaphores.
>
> > Sorry, but the behaviour is real.
>
> Oh, I'm sure the BSD kernel acts as you describe. But Mark's point is
> that Postgres never has more than one process waiting on any particular
> SysV semaphore, and so the problem doesn't really affect us.
>
> Or do you mean that the kernel wakes all processes sleeping on *any*
> SysV semaphore? That would be nasty :-(
To be clear, some behaviour that postgresql does with sysv semaphores
causes wakeups of many processes at once. i.e. if you have 20
clients, you will get up to 20 wakeups. I haven't studied the precise
cause of this, but it is empirically true. This is the scaling
problem I described, and it's what mux's patch addresses.
Kris
