Re: [HACKERS] Anyone interested in improving postgresql scaling?

Mark Kirkwood <markir@xxxxxxxxxxxxxxx> 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.

regards, tom lane
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