On Mon, Dec 12, 2011 at 8:33 AM, Craig Ringer <ringerc@xxxxxxxxxxxxx> wrote:
On 12/12/2011 08:42 AM, Stefan Keller wrote:You haven't specified your OS, Pg version, etc. This is important.
I'd like to clear the PostgreSQL cache (e.g. for benchmarking purposes).
And I'd like to preload all tuples of a table (say mytable_one) into the cache.
AFAIK there is no way to force all caches to be cleared in PostgreSQL
with an SQL command.
The only way to achieve this, seems to restart PG (server), which is
neither an option for benchmarking purposes nor for production.
Pg relies on the OS's disk cache, which it has no way to clear or control.
On Linux, check out the "drop_caches"No.
But:
1. Isn't it possible to achieve a kind-of cache clearing (in a
reliable way) by simply doing a "select * from mytable_two" given
mytable_two is at least as large as mytable_one (which is the one we
want to benchmark)?
If mytable_two is significantly larger than the system memory then maybe, and only maybe, you'll clear the cache. Nothing stops Pg from setting posix_fadvise(..., ..., ..., POSIX_FADV_SEQUENTIAL|POSIX_FADV_NOREUSE) to help the OS more efficently do the seqscan, though. Even if Pg doesn't do that, nothing stops the OS from figuring out Pg's intent and limiting how much it caches.Nope, again because Pg largely relies on the OS cache.
2. I assume that "select * from mytable_one" loads as much of the
tuples as it can into the cache. Are there better ways for preloading
the contents of a table?
The OS will *probably* cache the contents of mytable_one when you do a seqscan on it, but it might not, and it might be correct in not doing so.
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Craig Ringer
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For caching you can use pgmemcache a contrib module, however you need to change few thing in your application bcoz its not application transparent. For application transparency you can use infinite-cache.
I believe pgmemcache will do cache clearing, not sure whether its own cache or OS cache.
Very informative discussion on Caching in PostgreSQL archives.
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Regards,
Raghavendra
EnterpriseDB Corporation
