The 10% difference you see in clockticks and branches retired could be due to some kind of buffering of the disk files involved, but the reduced instructions retired are quite puzzling. Was the CPI value you show calculated by VTune or did you calculate it yourself? Usually when we a CPI that high its because there weren't enough samples to provide statistically meaningful data, but you have enough samples.

Yes, we regularly test rerunning an application to see how things change and we see the same phenomenon you did, that clockticks are usually less which we attribute to various kinds of buffering. But we also see the CPI remain reasonably constant.

I don't know of a way to shut off the O.S.'s buffereing, but I'm not really an O.S. expert.

May I ask you how did you tell the reason of 10% difference in clocktick and branches retired was the buffering of the disk files? Was it from the process view?

thanks.

VtuneUser

How big is the file you are using as an input to gzip also do you have calibration on or off?