Re: [Xen-users] Re: Xen Scheduler: Credit Scheduler ?

["Ulrich Windl" <ulrich.windl@xxxxxxxxxxxxxxxxxxxx>]

On 23 Nov 2006 at 11:33, Emmanuel Ackaouy wrote:

> It's a bit frustrating that we're not making progess isolating
> the problem here.
> 
> We still don't have any concrete evidence showing that the
> benchmark user processes or domain VCPU are or aren't runnable
> when you notice the "stall".

Stupid idea: just write a program that outputs the time of day at high 
resolution 
to some file. The run at least one such program in each domain (preferrably 
with 
synced clocks), then inspect the output for discontinuities.

> 
> It's also not clear what is the simplest scenario under which
> the problem can be reproduced. I tried reading your explanation
> about what happens with UP guests but I can't understand it.
> Can you clarify exactly what you are doing and in what order?

If it's not CPU-bound (the example uses little I/O), you could try something 
like 
copying as much data as 90% of your RAM from one file to another. Logically if 
a 
process in Dom0 is blocked on I/O, the scheduler can do little to help DomUs 
waiting for completion of that I/O. I suspect...

> 
> I threw around some ideas to get more data points and help
> debug this:
> - run spinners on the guest that consume CPU
>     "int main() { while (1); return 0; }" and run X copies (for X vcpus)
> - take scheduler traces (man xentrace)

A bad example, because that will either be optimized that it will execute in 
CPU's 
L1 cache, thus causing about no load at all (assuming a SMP system).

What about "find /usr -type f | xargs cat | gzip >/dev/null" instead. If you 
have 
plenty of room output to some real file. That's some load.

Regards,
Ulrich


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