Strange VFS performance problem

[Jeremy Allison]

On Wed, Feb 09, 2005 at 04:56:25PM -0800, Terry Griffin wrote:
> Hi all,
> 
> I'm having a very strange Samba VFS performance problem. Hoping
> you can provide some clues.
> 
> I've implemented a custom Samba VFS module. Functionally everything
> is fine and the module does what it's supposed to do.
> 
> With a Windows 2000 client the VFS module introduces an expected
> throughput hit on large writes in the range of 10-20% (over Gigabit
> Ethernet). But with a Linux CIFS client the throughput hit is more
> like 90%!
> 
> The Linux/CIFS and W2K client throughput numbers are similar in
> the case where the custom VFS module is not in use.
> 
> What about a VFS module could cause such drastically different
> results between a W2K client and a Linux/CIFS client? And is there
> something I can do to improve the Linux numbers? I've fiddled with
> directio and CIFSMaxBufSize options on the client side with no
> help. I've fiddled with all the usual tuning parameters on the Samba
> side, again with no improvement.
> 
> Linux (on both the CIFS client and Samba server) is Fedora Core 2
> with kernel 2.6.10. The Samba version is 3.0.10.

Try using cachegrind :

http://developer.kde.org/~sewardj/docs-2.2.0/cg_techdocs.html

Profile the fast and slow cases and look for differences. Use
smbd -i to look at one instance.

It's what I use to track down code performance problems in Samba.

Jeremy.