[Samba] more smbd CPU mystery

[Jeremy Allison]

On Fri, Oct 24, 2008 at 03:46:04PM -0700, Cochran, Wayne Owen wrote:
> Well I have determined that everytime someone logs in/logs out
> of a windows box in our lab *ALL* of the files in "My Directory"
> are copied from/to the file server to the local client. Needless to
> say this is retarded and needs to stop. The local sys admin needs
> to perform some windows voodoo to redirect this directory.
> 
> Still this brings the mystery as to why smbd would take up so
> much CPU. The work should mainly network and disk i/o bound
> (not CPU bound).  nfsd doesn't have this kind of bad CPU performance,
> why does smbd?
> 
> I had one of the users download a big file (which was being saved
> on the desktop). During this I was running top on the file server
> and noted it was continually soaking up 25% of the CPU:
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND      
> 12270 liffland  20   0 12748 4776 3628 S   24  0.1   1:02.36 smbd             
> 
> Why would disk/network traffic be so CPU heavy? Shouldn't this mostly
> be handled by a DMA controller?  Can anyone explain this.

You need to split out the CPU usage into user/kernel numbers.
Yes, smbd is using a lot of CPU here but if it's mostly in
kernel then it's just doing it's job. You *want* smbd to
be cpu bound, it's really easy to increase CPU by adding
more CPU than it is to increase network bandwidth
or disk i/o - that gets expensive.

Run vmstat to see what is using the CPU.

Jeremy.