Re: shared_buffers advice
| От | Dave Crooke |
|---|---|
| Тема | Re: shared_buffers advice |
| Дата | |
| Msg-id | ca24673e1003152217q117b0fd3q7d2d75ff20185c8b@mail.gmail.com обсуждение исходный текст |
| Ответ на | shared_buffers advice (Paul McGarry <paul@paulmcgarry.com>) |
| Ответы |
Re: shared_buffers advice
(Greg Smith <greg@2ndquadrant.com>)
|
| Список | pgsql-performance |
There seems to be a wide range of opinion on this .... I am new to PG and grew up on Oracle, where more SGA is always a good thing ... I know people who run Oracle on 2TB Superdome's with titanic SGA sizes to keep the whole DB in RAM. I'd be using a 40GB+ Oracle SGA on that box of yours.
A lot of the folks here say that there isn't much performance to be gained by giving PG's buffer cache the bulk of the RAM .... if you have the opportunity, it'd be interesting to test it both ways with a representative workload before going live. I for one would be very curious to see the results.
One caveat: the PG child processes can collectively use a lot of transient RAM, e.g. for sorts and vaccuming, depending on config and workload. If this causes swapping, or even uses up enough memory to effectively eliminate the OS buffer cache, it's going to hurt performance.
Cheers
Dave
A lot of the folks here say that there isn't much performance to be gained by giving PG's buffer cache the bulk of the RAM .... if you have the opportunity, it'd be interesting to test it both ways with a representative workload before going live. I for one would be very curious to see the results.
One caveat: the PG child processes can collectively use a lot of transient RAM, e.g. for sorts and vaccuming, depending on config and workload. If this causes swapping, or even uses up enough memory to effectively eliminate the OS buffer cache, it's going to hurt performance.
Cheers
Dave
On Wed, Mar 10, 2010 at 8:28 PM, Paul McGarry <paul@paulmcgarry.com> wrote:
Hi there,
I'm after a little bit of advice on the shared_buffers setting (I have
read the various docs on/linked from the performance tuning wiki page,
some very helpful stuff there so thanks to those people).
I am setting up a 64bit Linux server running Postgresql 8.3, the
server has 64gigs of memory and Postgres is the only major application
running on it. (This server is to go alongside some existing 8.3
servers, we will look at 8.4/9 migration later)
I'm basically wondering how the postgresql cache (ie shared_buffers)
and the OS page_cache interact. The general advice seems to be to
assign 1/4 of RAM to shared buffers.
I don't have a good knowledge of the internals but I'm wondering if
this will effectively mean that roughly the same amount of RAM being
used for the OS page cache will be used for redundantly caching
something the Postgres is caching as well?
IE when Postgres reads something from disk it will go into both the OS
page cache and the Postgresql shared_buffers and the OS page cache
copy is unlikely to be useful for anything.
If that is the case what are the downsides to having less overlap
between the caches, IE heavily favouring one or the other, such as
allocating shared_buffers to a much larger percentage (such as 90-95%
of expected 'free' memory).
Paul
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