On Tue, Apr 10, 2007 at 10:23:42AM -0400, Tom Lane wrote:
> Mark Kirkwood <markir@paradise.net.nz> writes:
> > Kris Kennaway wrote:
> >> If so, then your task is the following:
> >>
> >> 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. i.e. there is a thundering herd
> >> wakeup situation which destroys performance at high loads. Fixing
> >> this will involve replacing the wakeup() calls with appropriate
> >> amounts of wakeup_one().
>
> > I'm forwarding this to the pgsql-hackers list so that folks more
> > qualified than I can comment, but as I understand the way postgres
> > implements locking each process has it *own* semaphore it waits on -
> > and who is waiting for what is controlled by an in (shared) memory hash
> > of lock structs (access to these is controlled via platform Dependant
> > spinlock code). So a given semaphore state change should only involve
> > one process wakeup.
>
> 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.
Kris
