[Gllug] Profiling - where is all the time going?

[Daniel P. Berrange]

On Tue, Jul 20, 2004 at 11:10:51AM +0100, John Winters wrote:
> Hi all,
> 
> I'm experimenting with profiling a compute intensive program.  It does
> practically no i/o and it isn't memory bound - it just thinks hard.
> 
> I've compiled and linked with -pg and I seem to be getting a reasonably
> meaningful output except for one thing.  If I let the program run for 60
> seconds and then examine the profiling information I'm told that the
> cumulative time used by all the functions is 10.77 seconds.  Where could
> the other 49 seconds be going?

-pg is pretty limited level of profiling really. I'd recommend
installing OProfile, which uses CPU performance counters to 
profile across both user & kernel space (it will associate kernel
space time which the process responsible). It has very low impact
on program runtime compared to -pg, and can track various differnet
metrics depending on your processor. For example, simple counter
is 'time during which CPU is not halted' - ie running time. More
advanced counters can track 2nd/3rd level cache hits/misses,
branch prediction success rate, all sort of funky stuff....

Dan.
-- 
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