CPU usage of VTune on Linux

[Kevin_T_Intel]

Hi,

I am doing some sampling on a Pentium 2.4GHz desktop, with RH Ent Linux 3.0. I find that when I do sampling with the following

sampling interval = 1ms

buff size=20000KB

sample after clockticks =2000

sample after instructions retired=2000

When I run Vtune, I find that my CPU is going up to 100%. I'm afraid that the processes I am samplling is being starved of CPU and hence my measurments inaccurate. Is it normal for VTune to bring CPU usage up to 100% ? does it sort of "nice" itself so the processes being measured are measured accurately ? Is the only answer to trade off sample after clockticks figure with CPU usage ? Remote data collection is not supported with my Kernel.

any advice welcome,

rgds,

ktraynor (Intel Employee)

[TimP]

I suppose such high sample after values could make the VTune CPU usage intrusive. Are you trying to find out why your application doesn't normally have high CPU usage? I guess it's not likely that events which you can't see with normal sample after values would explain low CPU usage. The default values are coupled; since the default sample after values are usually OK for the default 20 second sampling interval, you shouldn't need to boost them by a factor of 5 unless your sampling interval is under 5 seconds. If your application takes around 4 seconds elapsed time, and significantly less CPU time, VTune may not help a great deal.
I'll sign off at this point; I have difficulty profiling my applications in 10 minute sessions, so 4 second sessions aren't my specialty.

[David_A_Intel1]

Yes, Tim is correct. You should use the default configuration for Clockticks and Instructions Retired. If you increase the frequency, as you have, it will significantly impact the system.