Re: High SYS CPU - need advise

On Tue, Nov 20, 2012 at 9:05 AM, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
> On Tue, Nov 20, 2012 at 10:50 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>>
>> I wouldn't expect so. Increasing shared_buffers should either fix
>> free list lock contention, or leave it unchanged, not make it worse.
>
> AIUI, that is simply not true (unless you raise it to the point you're
> not churning them). I'm looking at StrategyGetBuffer() for non-scan
> cases. It locks "BufFreelistLock" then loops the free list, and, if
> it finds nothing, engages a clock sweep.

The freelist should never loop. It is written as a loop, but I think
there is currently no code path which ends up with valid buffers being
on the freelist, so that loop will never, or at least rarely, execute
more than once.

> Both of those operations are
> dependent on the number of buffers being managed and so it's
> reasonable to expect some workloads to increase contention with more
> buffers.

The clock sweep can depend on the number of buffers begin managed in a
worst-case sense, but I've never seen any evidence (nor analysis) that
this worst case can be obtained in reality on an ongoing basis. By
constructing two pathological workloads which get switched between, I
can get the worst-case to happen, but when it does happen the
consequences are mild compared to the amount of time needed to set up
the necessary transition. In other words, the worse-case can't be
triggered often enough to make a meaningful difference.

Cheers,

Jeff