On 4/28/14, 8:04 AM, Robert Haas wrote:
On Mon, Apr 21, 2014 at 6:38 PM, Jim Nasby <j...@nasby.net> wrote:
I feel that if there is no memory pressure, frankly it doesnt matter much
about what gets out and what not. The case I am specifically targeting is
when the clocksweep gets to move about a lot i.e. high memory pressure
workloads. Of course,  I may be totally wrong here.


Well, there's either memory pressure or there isn't. If there isn't then
it's all moot *because we're not evicting anything*.

I don't think that's really true.  A workload can fit within
shared_buffers at some times and spill beyond it at others.  Every
time it fits within shared_buffers for even a short period of time,
the reference count of any buffer that's not ice-cold goes to 5 and we
essentially lose all knowledge of which buffers are relatively hotter.
  Then, when we spill out again, evictions are random.

That's a separate problem, but yes, just because we're not evicting something 
doesn't mean we can end up with every buffer marked as equally important.

OSes handle this by splitting pages between active and inactive, and 
maintaining a relative balance between the two (actually a bit more complex 
because there's a separate inactive/dirty pool).

In our case this could maybe be handled by simply not incrementing counts when 
there's no eviction... but I'm more a fan of separate polls/clocks, because 
that means you can do things like a LFU for active and an LRU for inactive.
--
Jim C. Nasby, Data Architect                       j...@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net


--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Reply via email to