Re: Bgwriter strategies

Heikki Linnakangas wrote:
> I scheduled a test with the moving average method as well, we'll see how 
> that fares.

No too well :(.

Strange. The total # of writes is on par with having bgwriter disabled, 
but the physical I/O graphs show more I/O (on par with the aggressive 
bgwriter), and the response times are higher.

I just noticed that on the tests with the moving average, or the simple 
"just enough" method, there's a small bump in the CPU usage during the 
ramp up period. I believe that's because bgwriter scans through the 
whole buffer cache without finding enough buffers to clean. I ran some 
tests earlier with unpatched bgwriter tuned to the maximum, and it used 
~10% of CPU, which is the same level that the bump rises to. 
Unfortunately I haven't been taking pg_buffercache snapshots until after 
the ramp up; it should've shown up there.

I've been running these test with bgwriter_delay of 10 ms, which is 
probably too aggressive. I used that to test the idea of starting the 
scan from where it left off, instead of always starting from clock hand.

If someone wants to have a look, the # of writes are collected to a 
separate log file in <test number>/server/buf_alloc_stats.log. There's 
no link to it from the html files. There's also summary snapshots of 
pg_buffercache every 30 seconds in <test number>/server/bufcache.log.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com