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Kevin_T_Intel
Employee
99 Views

CPU usage of VTune on Linux

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)
0 Kudos
2 Replies
TimP
Black Belt
99 Views

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
Employee
99 Views

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.
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