On 2/12/16 9:55 AM, Robert Haas wrote:
I think it's important to spend time and energy figuring out exactly
what the problems with our current algorithm are.  We know in general
terms that usage counts tend to converge to either 5 or 0 and
therefore sometimes evict buffers both at great cost and almost

Has anyone done testing on the best cap to usage count? IIRC 5 was pulled out of thin air. Actually, I don't recall ever seeing a clock sweep that supported more than a single bit, though often there are multiple 'pools' a buffer could be in (ie: active vs inactive in most unix VMs).

If you have a reasonable amount of 1 or 0 count buffers then this probably doesn't matter too much, but if your working set is significantly higher than shared buffers then you're probably doing a LOT of full sweeps to try and get something decremented down to 0.

randomly.  But what's a lot less clear is how much that actually hurts
us given that we are relying on the OS cache anyway.  It may be that
we need to fix some other things before or after improving the buffer
eviction algorithm before we actually get a performance benefit.  I
suspect, for example, that a lot of the problems with large
shared_buffers settings have to do with the bgwriter and checkpointer
behavior rather than with the buffer eviction algorithm; and that
others have to do with cache duplication between PostgreSQL and the
operating system.  So, I would suggest (although of course it's up to

It would be nice if there was at least an option to instrument how long an OS read request took, so that you could guage how many requests were coming from the OS vs disk. (Obviously direct knowledge from the OS is even better, but I don't think those APIs exist.)

you) that you might want to focus on experiments that will help you
understand where the problems are before you plunge into writing code
to fix them.

Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com

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