Re: [Maybe Spam] Re: memory cache problem

[Al Tobey <albert tobey priority-health com>]

On Wed, 2004-09-15 at 11:59, John Haxby wrote:
> Mark Dadgar wrote:
> 
> > This really, really sucks.  And hopefully will get fixed one of these 
> > days, as it's going to be a real problem soon when I go into 
> > production.  Paging out the SQL server is a very bad idea.
I think recent versions of MySQL can be instructed to mlock() it's
memory so it can't be swapped.   PostgreSQL probably has this as well.  
Oracle, I think, does this by default.

> 
> Ahh.   I see.  I do remember something in the archives about tuning the 
> cache parameters for precisely this reason -- another poster (sorry, the 
> message has been deleted) suggested doing exactly the same thing.   I 
> agree that you don't want the SQL server paging out even if it appears 
> to be idle.   I wish I could remember something else about the tuning, 
> but there was definitely something about SQL servers and cache tuning on 
> the list a little while back.

> jch
Grab the Documentation/filesystems/proc.txt file from a 2.6 kernel
distribution - the one in the RH kernel source is severely outdated. 
Mainly, you'll want to tweak vfs_cache_pressure (vm.vfs_cache_pressure
in /etc/sysctl.conf) on RHEL.   It appears that some of the tunables for
percentage of memory to use for caching are no longer available in RHEL
and 2.6 kernels.

vfs_cache_pressure
------------------

Controls the tendency of the kernel to reclaim the memory which is used for
caching of directory and inode objects.

At the default value of vfs_cache_pressure=100 the kernel will attempt to
reclaim dentries and inodes at a "fair" rate with respect to pagecache and
swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
to retain dentry and inode caches.  Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes.




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