Re: System CPU increasing on idle 2.6.36

[Simon Kirby <sim@hostway.ca>]

On Wed, Dec 08, 2010 at 01:25:05PM -0800, Simon Kirby wrote:

> Possibly related to the flush-processes-taking-CPU issues I saw
> previously, I thought this was interesting.  I found a log-crunching box
> that does all of its work via NFS and spends most of the day sleeping. 
> It has been using a linearly-increasing amount of system time during the
> time where is sleeping.  munin graph:
> 
> http://0x.ca/sim/ref/2.6.36/cpu_logcrunch_nfs.png
>...
> Known 2.6.36 issue?  This did not occur on 2.6.35.4, according to the
> munin graphs.  I'll try 2.6.37-rc an see if it changes.

So, back on this topic,

It seems that system CPU from "flush" processes is still increasing
during and after periods of NFS activity, even with 2.6.37-rc5-git4:
        
        http://0x.ca/sim/ref/2.6.37/cpu_nfs.png

Something is definitely going on while NFS is active, and then keeps
happening in the idle periods.  top and perf top look the same as in
2.6.36.  No userland activity at all, but the kernel keeps doing stuff.

I could bisect this, but I have to wait a day for each build, unless I
can come up with a way to reproduce it more quickly.  The mount points
for which the flush processes are active are the two mount points where
the logs are read from, rotated, compressed, and unlinked, and where the
reports are written, running in parallel under an xargs -P 15.

I'm pretty sure the only syscalls that are reproducing this are read(),
readdir(), lstat(), write(), rename(), unlink(), and close().  There's
nothing special happening here...

Simon-