Horton

Re: Clock sweep not caching enough B-Tree leaf pages?

Robert Haas <robertmhaas@gmail.com>

View thread View on postgr.es
From: Robert Haas <robertmhaas@gmail.com>
To: Jim Nasby <jim@nasby.net>
Cc: Atri Sharma <atri.jiit@gmail.com>, Bruce Momjian <bruce@momjian.us>, Peter Geoghegan <pg@heroku.com>, Greg Stark <stark@mit.edu>, Ants Aasma <ants@cybertec.at>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2014-04-28T13:04:51Z
Lists: pgsql-hackers 
On Mon, Apr 21, 2014 at 6:38 PM, Jim Nasby <jim@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.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Replace the BufMgrLock with separate locks on the lookup hashtable and