Re: 2.4.25 - large inode_cache

[Jakob Oestergaard]

On Thu, Feb 26, 2004 at 10:08:23AM -0300, Marcelo Tosatti wrote:
...
> >
> > free output is this:
> >              total       used       free     shared    buffers  cached
> > Mem:        515980     506464       9516          0    2272      19204
> > -/+ buffers/cache:     484988      30992
> > Swap:      1951856       7992    1943864
> 
> This should be normal behaviour -- the i/d caches grew because of file
> system activitity. This memory will be reclaimed in case theres pressure.

But how is "pressure" defined?

Will a heap of busy knfsd processes doing reads or writes exert
pressure?   Or is it only local userspace that can pressurize the VM (by
either anonymously backed memory or file I/O).

This server happily serves large home directories over NFS, at really
poor speeds.  It will happily serve tens or hundreds of gigabytes, read
and write, over the course of a day, and *still* only cache about 100MB
NFS to/from the server is slow. It's common to see 10 knfsd processes in
D state while vmstat tells me the array works with about 4-6MB/sec
sustained throughput (where hdparm -t would give me more than 70MB/sec
on the md device).

The files read and written are commonly in the 20-60 MB range, so it's
not just because I'm loading the server with small seeks. Many files are
read multiple times within a few minutes, so the cache usage of 100MB is
completely bogus the way that I see it - but maybe there's just
something I don't know about the caching?   :)

> 
> Is the behaviour different from previous 2.4 or 2.6 kernels?

I never investigated the slabinfo on earlier 2.4. But the performance on
this server has been "under expectations" for as long as I can remember.
So, from the performance experience on this server I would say that
2.4.25 is not any worse than older kernels.

Since this is a production system I have been reluctant to jump on the
2.6 wagon - but my other experiences with 2.6.X have been good, so I'm
probably going to soften up and give it a try in a not too distant
future.

However, if this dcache/icache problem is well known and is (or at least
should be) solved in 2.6, then I can do the test this weekend.

Any enlightenment or suggestions are greatly appreciated :)

Thanks,

 / jakob


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