[Planetlab-users] cpu freezes question

[David E. Eisenstat]

On Sat, 25 Aug 2007, Robert P Ricci wrote:

> Thus spake Roger Pack on Thu, Aug 23, 2007 at 08:49:32PM -0600:
>> I have encountered a few cpu freezes on processes, and, wondering if it 
>> was just me, found this online ... On PlanetLab, however, high CPU load 
>> occasionally causes processes to freeze for several seconds, long 
>> enough for several successive pings to time out. Chord then incorrectly 
>> declared peers offline and potentially misrouted messages....
>>
>> My question is how long does these normally last?  I've had some in the 
>> 10-20 second range--is that normal?  Thanks!
>
> I've definitely experienced behavior like this, though not as severe as 
> you seem to have. We did some experiments to measure CPU starvation on 
> PlanetLab, and the longest period we saw was around half a second. I 
> could make this code available to you if you'd like to do your own 
> study. (Some results from our study are in the third paper referenced 
> below.)
>
> I've also experienced freezes that seemed to result from extremely long 
> disk I/O latency. Offhand, I would say that those freezes tend to be 
> longer than the periods of CPU starvation, but I haven't directly 
> studied them.

This has been my experience as well. The only times I've seen 10-20 second 
freezes is when my program was blocked on disk I/O or a DNS lookup. If you 
have a CPU hog in your slice, that probably doesn't help either: if your 
sliver runs out of its CPU tokens, ALL of its processes are suspended 
until it's back over threshold. I haven't been able to produce delays that 
long with this effect, though.

Another paper that discusses these issues is

"Using PlanetLab for Network Research: Myths, Realities, and Best 
Practice". Neil Spring, Larry Peterson, Andy Bavier, Vivek S. Pai, In 
Operating Systems Review Vol 40, No 1, Jan 2006

-David