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* Taking the cache out of the equation?
@ 2009-06-09 17:51 Rob Sargent <robjsargent@gmail.com>
2009-06-12 22:12 ` Re: Taking the cache out of the equation? Erik Jones <ejones@engineyard.com>
0 siblings, 1 reply; 3+ messages in thread
From: Rob Sargent @ 2009-06-09 17:51 UTC (permalink / raw)
To: pgsql-sql
Caching helps a *lot* and I'm thankful for that but I would like to take
it out of the picture as I massage my queries for better performance.
Naturally the first invocation of the query cannot take advantage of the
cache and these queries would normally only be called once for the same
target data. What tricks are there to flush, ignore, circumvent the
caching boost? (Especially in the production environment.)
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Taking the cache out of the equation?
2009-06-09 17:51 Taking the cache out of the equation? Rob Sargent <robjsargent@gmail.com>
@ 2009-06-12 22:12 ` Erik Jones <ejones@engineyard.com>
2009-06-12 22:25 ` Re: Taking the cache out of the equation? Greg Stark <gsstark@mit.edu>
0 siblings, 1 reply; 3+ messages in thread
From: Erik Jones @ 2009-06-12 22:12 UTC (permalink / raw)
To: Rob Sargent <robjsargent@gmail.com>; +Cc: pgsql-sql
On Jun 9, 2009, at 10:51 AM, Rob Sargent wrote:
> Caching helps a *lot* and I'm thankful for that but I would like to
> take it out of the picture as I massage my queries for better
> performance. Naturally the first invocation of the query cannot
> take advantage of the cache and these queries would normally only be
> called once for the same target data. What tricks are there to
> flush, ignore, circumvent the caching boost? (Especially in the
> production environment.)
Why on earth would you want your queries to always go to disk?
Erik Jones, Database Administrator
Engine Yard
Support, Scalability, Reliability
866.518.9273 x 260
Location: US/Pacific
IRC: mage2k
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Taking the cache out of the equation?
2009-06-09 17:51 Taking the cache out of the equation? Rob Sargent <robjsargent@gmail.com>
2009-06-12 22:12 ` Re: Taking the cache out of the equation? Erik Jones <ejones@engineyard.com>
@ 2009-06-12 22:25 ` Greg Stark <gsstark@mit.edu>
0 siblings, 0 replies; 3+ messages in thread
From: Greg Stark @ 2009-06-12 22:25 UTC (permalink / raw)
To: Erik Jones <ejones@engineyard.com>; +Cc: Rob Sargent <robjsargent@gmail.com>; pgsql-sql
On Sat, Jun 13, 2009 at 12:12 AM, Erik Jones<ejones@engineyard.com> wrote:
>
> On Jun 9, 2009, at 10:51 AM, Rob Sargent wrote:
>
>> Caching helps a *lot* and I'm thankful for that but I would like to take
>> it out of the picture as I massage my queries for better performance.
>> Naturally the first invocation of the query cannot take advantage of the
>> cache and these queries would normally only be called once for the same
>> target data. What tricks are there to flush, ignore, circumvent the
>> caching boost? (Especially in the production environment.)
>
> Why on earth would you want your queries to always go to disk?
I think he answered that in the original message -- to better
represent the real workload.
Unfortunately there isn't really a good answer. On Linux you can echo
1 > /proc/sys/vm/drop_caches but that doesn't affect the postgres
shared buffers and worse, it does affect other buffers that probably
would still be cached.
The best answer is usually to build a test configuration large enough
that it has similar cache effects as your production environment. Then
test random values and repeat the test many times to avoid any random
fluctuations.
--
greg
http://mit.edu/~gsstark/resume.pdf
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2009-06-09 17:51 Taking the cache out of the equation? Rob Sargent <robjsargent@gmail.com>
2009-06-12 22:12 ` Erik Jones <ejones@engineyard.com>
2009-06-12 22:25 ` Greg Stark <gsstark@mit.edu>
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