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Thread: system cache and buffer cache

system cache and buffer cache

From
Praveen Kumar N
Date:
Hi,    can anybody explain me what is the difference between system cache 
and buffer cache?

I found that keywords in PostgreSql FAQ 
http://www.postgresql.org/docs/faqs.FAQ_DEV.html#item2.1

thanks in advance.

Regards,
--  N Praveen Kumar
    Imagination is more important than knowledge...                                --Albert Einstein

Re: system cache and buffer cache

From
Heikki Linnakangas
Date:
Praveen Kumar N wrote:
> Hi,
>     can anybody explain me what is the difference between system cache 
> and buffer cache?
>
> I found that keywords in PostgreSql FAQ 
> http://www.postgresql.org/docs/faqs.FAQ_DEV.html#item2.1

System cache is a per-row cache of system catalog tables. It's used to 
speed up lookup of things like function names. It's implemented in 
src/backend/utils/cache/syscache.c

Buffer cache is the cache managed by the buffer manager, that caches any 
blocks from any relation used in the system. All access to relations go 
through the buffer cache, using ReadBuffer/ReleaseBuffer (etc.) calls. 
It's implemented in src/backend/storage/buffer/bufmgr.c

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

Re: system cache and buffer cache

From
Praveen Kumar N
Date:
Buffer cache is implemented using bufferpool right(I mean in the main 
memory).how about system cache? Can we control the size of system cache?

On Tue, 19 Sep 2006, Heikki Linnakangas wrote:

> System cache is a per-row cache of system catalog tables. It's used to speed 
> up lookup of things like function names. It's implemented in 
> src/backend/utils/cache/syscache.c
>
> Buffer cache is the cache managed by the buffer manager, that caches any 
> blocks from any relation used in the system. All access to relations go 
> through the buffer cache, using ReadBuffer/ReleaseBuffer (etc.) calls. It's 
> implemented in src/backend/storage/buffer/bufmgr.c
>
>

--  N Praveen Kumar Btech-IV CSE IIIT,Hyd AP,India
    Imagination is more important than knowledge...                                --Albert Einstein

Re: system cache and buffer cache

From
Heikki Linnakangas
Date:
Praveen Kumar N wrote:
> Buffer cache is implemented using bufferpool right(I mean in the main 
> memory).

Yes. It's in shared memory, size is controlled by the shared_buffers 
configuration parameter.

> how about system cache? Can we control the size of system cache?

It's in backend-private memory. I don't remember how it's sized.

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

Re: system cache and buffer cache

From
Alvaro Herrera
Date:
Praveen Kumar N wrote:
> Hi,
>     can anybody explain me what is the difference between system cache 
> and buffer cache?
> 
> I found that keywords in PostgreSql FAQ 
> http://www.postgresql.org/docs/faqs.FAQ_DEV.html#item2.1

Another important cache is the "relation cache", relcache for short,
which is also stored in local memory.

One important point about these (relcache, syscache, catcaches = catalog
caches) is that they are invalidated using the "sinval" system, which
passes messages from one backend to all others via a queue in shared
memory.   The buffer cache needs no such thing, precisely because it
lives in shared memory.  But it needs to be protected by locks.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

Re: system cache and buffer cache

From
Simon Riggs
Date:
On Tue, 2006-09-19 at 14:59 +0100, Heikki Linnakangas wrote:
> Praveen Kumar N wrote:

> > how about system cache? Can we control the size of system cache?
> 
> It's in backend-private memory. I don't remember how it's sized.

It's fixed size: syscache caches a predefined set of catalog info.

--  Simon Riggs              EnterpriseDB   http://www.enterprisedb.com