Does Cyrus benefit greatly from increased FS buffer cache?

[Blake Hudson]

We're able to utilize ~ 4GB of cache on a 32bit  installation of 
Cyrus/Postfix (we're using ext3 - dunno if it has a different cache 
limitation compared to reiser). This installation serves ~15k mailboxes 
and has between 250 and 500 active POP/IMAP sessions at any given time.

Unfortunately, we do not currently make use of the full 24GB RAM in 
these machines. Perhaps with a 64 bit installation would resolve that... 
anyway, here are the mem stats.

----
             total       used       free     shared    buffers     cached
Mem:      24906720    9973956   14932764          0     298412    4513164
-/+ buffers/cache:    5162380   19744340
Swap:      1052248          0    1052248
----

If we are able to make use of 10GB (half of which is cache) on a 
similarly loaded machine, you should be able use just as much (if not 
more on a 64bit installation).

--Blake

-------- Original Message  --------
Subject: Does Cyrus benefit greatly from increased FS buffer cache?
From: John Widera <jwidera at oakton.edu>
To: info-cyrus at lists.andrew.cmu.edu
Cc: data center <datacenter at oakton.edu>
Date: Tuesday, April 14, 2009 3:14:39 PM
> Hi,
>
> Last week we posted to the list for info about running Cyrus on 64-bit
> RHEL.  But we also needed to ask a more direct question about caching,
> though, and didn't.  So we are now...
>
> That q. is, does Cyrus actually benefit from large amounts of memory in an
> environment with just a few thousand users ( <1000 simultaneous sessions)
> and, say, up to 750GB of mail spool?
>
> Our plan is to throw 12-16GB at it, with the purpose of vastly increasing
> the FS buffer cache (and decreasing I/O).  Or, will that just be a waste
> of RAM?
>
> Some indications are that, yes, it does improve performance notably:
>
> http://blog.fastmail.fm/2007/09/21/reiserfs-bugs-32-bit-vs-64-bit-kernels-cache-vs-inode-memory/
>
> Anyone have any specific thoughts?  Is there any other benefit we might
> see from large memory allocation in 64-bit architecture?
>
> Thanks,
>
> John Widera
> Oakton Community College
> Des Plaines and Skokie, IL
>
> ----
> Cyrus Home Page: http://cyrusimap.web.cmu.edu/
> Cyrus Wiki/FAQ: http://cyrusimap.web.cmu.edu/twiki
> List Archives/Info: http://asg.web.cmu.edu/cyrus/mailing-list.html
>