On Thu, Feb 26, 2004 at 06:06:06PM -0500, Christopher Weimann wrote:
> On 02/26/2004-11:16AM, Dror Matalon wrote:
> > > 
> > > effective_cache_size changes no cache settings for postgresql, it simply 
> > > acts as a hint to the planner on about how much of the dataset your OS / 
> > > Kernel / Disk cache can hold.
> > 
> > I understand that. The question is why have the OS, in this case FreeBsd
> > use only 200 Megs for disk cache and not more. Why not double the
> > vfs.hibufspace  to 418119680 and double the effective_cache_size to 51040.
> > 
> FreeBSD uses ALL ram that isn't being used for something else as
> its disk cache.  The "effective_cache_size" in the PostGreSQL config
> has no effect on how the OS chooses to use memory, it is just hint
> to the PostGreSQL planner so it can guess the the likelyhood of
> what it is looking for being in the cache.

Let me try and say it again. I know that setting effective_cache_size
doesn't affect the OS' cache. I know it just gives Postgres the *idea*
of how much cache the OS is using. I know that. I also know that a
correct hint helps performance.

I've read Matt Dillon's discussion about the freebsd VM at
http://www.daemonnews.org/200001/freebsd_vm.html and I didn't see him
saying that Freebsd uses all the free RAM for disk cache. Would you care
to provide a URL pointing to that?

Assuming you are correct, why has the ongoing recommendation been to use
hibufspace/8192 as the effective_cache_size? Seems like it would be
quite a bit more on machines with lots of RAM.



Dror Matalon
Zapatec Inc 
1700 MLK Way
Berkeley, CA 94709

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