Re: High 50us+ latencies in the process signal handling path

[ankita@in.ibm.com (Ankita Garg)]

Hi Steve,

On Thu, Oct 18, 2007 at 09:38:44AM -0400, Steven Rostedt wrote:
> 
> --
> When I'm asked what language is my mother tongue,
> I simply answer "C".
> 
> On Thu, 18 Oct 2007, Ankita Garg wrote:
> 
> > > > Any thoughts on where in the code could these large latencies be
> > > > attributed to?
> > >
> > > If the max latency is usually the first one, it's could be the page
> > > fault the signal handler is taking the first time it is executed.
> >
> > Thanks for looking into this. Attaching the entire log from the testrun.
> > It indicates that in each run of the 10000 iterations, there are about 100+
> > failures, with large iterations. Also, on intrumenting the testcode to
> > find out the iterations which see large latencies, I see that as the
> > number of iterations increase, the latencies increase.
> 
> OK, this doesn't look like page faulting issues. But it is still a good
> idea to add mlock to any RT tests.
> 
> Could this possible be simply cache misses that cause this?  There's a bit
> of code to send a signal. What would the impact of cold cache be on this?
> 
> One way is to see the mininum run of running with cached disabled. Not
> sure if there's any way to disable cache via a kernel command line. Of
> course that would make the system very slow to boot ;-)
>

The machines I am working on does not support disabling caches via BIOS
and as Documentation/memory.txt suggests, BIOS seems to be the only way.

Also, mlocking did not make any difference in the latencies.

> -- Steve

-- 
Regards,
Ankita Garg (ankita@in.ibm.com)
Linux Technology Center
IBM India Systems & Technology Labs, 
Bangalore, India