Disk caching
Hi to all.
I am trying to see how PostgreSQL performance changes on the basis of
work_mem. So, I am going to execute the 22 queries of TPCH
(http://www.tpc.org/tpch/) again and again, each time for a different
value of work_mem.
Since I am interested just in work_mem variations, I should prevent each
query to take advantages from revious executions of the 22 queries them
selves. For example, taking cache advantages. So, taking into account
that the 22 queries are those http://pastebin.com/7Dg50YRZ and are
executed on tables of hundreds of MB and
1) Is it sufficient to run change the values of work_mem through psql
and running the queries again without restarting postgres?
2) Or, should I restart postgres?
3) Or, shoud I restart the machine each time I execute the 22 queries?
Thanks for your time.
Regards.
Manolo.
mac_man2008@yahoo.it wrote:
Since I am interested just in work_mem variations, I should prevent
each query to take advantages from revious executions of the 22
queries them selves. For example, taking cache advantages.
work_mem has nothing to do with disk caching; it controls whether larger
sorts are done using disk or memory. The main parameter that controls
the size of the disk cache is shared_buffers. If you found the
descriptions in the manual confusing,
http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server is another
guide to what the settings mean that may help you out.
If you want to eliminate caching effects from your test, you will need
to both restart the database server and flush the operating system
cache. You can do this easily on Linux:
http://linux-mm.org/Drop_Caches but it's not as simple to do on most
other platforms without rebooting.
P.S. The right list for this discussion if you want to talk about it
more is pgsql-performance. This one is focused at things involving
PostgreSQL development, not questions related to using the database.
--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com www.2ndQuadrant.us