On 2/3/17 6:20 PM, Andres Freund wrote:
>> - The ringbuffers in shared buffers can be problematic. One possible way of
>> solving that is to get rid of ringbuffers entirely and rely on different
>> initial values for usage_count instead, but that's not desirable if it just
>> means more clock sweep work for backends.
> I'm not quite sure which ringbuffer you're referring to here? If to the
> new one, why is it problematic?
No, I mean the non-default BufferAccessStrategy's.
>> FWIW, I started looking into this stuff because of a customer system where
>> shared buffers is currently ~4x larger than the cluster, yet there's a
>> non-trivial amount of buffers being written by backends.
> That's probably not related to bgwriter then. If there's free pages
> available (which there have to be if s_b is bigger than the cluster)
> there'll be no clock sweep / victim buffers. I strongly suspect that
> you're seeing backend writes due to the write ringbuffers, e.g. by
Right.
> seqscans, copy, vacuum - those won't be affected in either case.
Yeah, my idea there is that if there was a separate process responsible
for the clock sweep then perhaps instead of ring buffers we could simply
use different initial values for usage_count to control how likely
things were to get evicted. Trying to do that with all the backends
needing to run the clock would presumably be a really bad idea though.
> I'd suggest installing pg_stat_statements and enabling track_io_timings
> - that'll tell you which statements triggered backend writes and how
> long they took.
Good idea.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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