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Re: Slab memory usage

To: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Subject: Re: Slab memory usage
From: "Josef 'Jeff' Sipek" <jeffpc@xxxxxxxxxxxxxx>
Date: Mon, 27 Apr 2009 04:40:35 -0400
Cc: xfs@xxxxxxxxxxx
In-reply-to: <200904262351.22970@xxxxxx>
References: <73EE3FB2-381F-43F1-82C1-FA4C020E7C02@xxxxxxxxxxx> <49F260F1.4030503@xxxxxxxxxxx> <200904262351.22970@xxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
On Sun, Apr 26, 2009 at 11:51:22PM +0200, Michael Monnerie wrote:
> On Samstag 25 April 2009 Eric Sandeen wrote:
> > *from Documentation/sysctl/vm.txt:
> >
> > 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.
> So if I decrease it, lets say to 60, Linux prefers to remember 
> files/dirs over their content. An increase to 150 would mean Linux 
> prefers to keep file contents over dirs/files?

Yep, that's right.

> If so, I think for a fileserver for many users accessing many 
> dirs/files, I'd prefer a lower value, in order to prevent searching. 
> Disk contents can be read fast, with all the read-ahead caching of 
> disks/controllers and Linux itself, but the scattered dirs take loooong 
> to scan sometimes. (Example: a foto collection with 50.000 files in many 
> dirs). Am I right?

Approximate answer is: it depends on the frequency of meta-data reads vs.
data reads. Your reasoning is fine if whoever access the photo collection
does not frequently read the photos themselves.

Best answer is: benchmark it with the exact workload you have to deal with

Josef 'Jeff' Sipek.

Intellectuals solve problems; geniuses prevent them
                - Albert Einstein

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