Re: Fix CheckpointStartLock starvation

Tom Lane wrote:
> Heikki Linnakangas <heikki@enterprisedb.com> writes:
>> On a busy system, checkpoint could be starved while queuing for the
>> CheckpointStartLock. To avoid that, get rid of CheckpointStartLock and
>> instead set a flag in PGPROC struct when a commit starts. After
>> computing the REDO ptr, checkpoint waits for all backends that had that
>> flag set to finish their commits. This eliminates the same race
>> condition the CheckpointStartLock was there for, without the risk of
>> starvation.
>
> Applied with some revisions --- I did not see the point of forcing
> checkpoint to wait till the transaction was fully out of its commit;
> we only need it to wait till clog is updated.

Thanks! I'll rerun my test with this.

There was no particular reason to delay, but I figured it doesn't really
matter either way since checkpoints are not in a hurry.

> The procarray code seemed overly complicated too.

Yeah, it was a bit ugly. I wanted to doing the O(n^2) looping while
holding the ProcArrayLock. I doubt it's a problem in practice since it's
only done during a checkpoint and n should be small, but I'll stick to
that excuse.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com