Il 03/06/2013 08:38, Gleb Natapov ha scritto: > On Mon, Jun 03, 2013 at 08:33:13AM +0200, Paolo Bonzini wrote: >> Il 02/06/2013 17:32, Gleb Natapov ha scritto: >>> On Thu, May 30, 2013 at 07:43:07PM +0200, Paolo Bonzini wrote: >>>> This patch includes two fixes for SB: >>>> >>>> * the 3rd fixed counter ("ref cpu cycles") can sometimes report >>>> less than the number of iterations >>>> >>> Is it documented? It is strange for "architectural" counter to behave >>> differently on different architectures. >> >> It just counts the CPU cycles. If the CPU can optimize the loop better, >> it will take less CPU cycles to execute it. >> > We should try and change the loop so that it will not be so easily optimized. > Making the test succeed if only 10% percent of cycles were spend on a loop > may result in the test missing the case when counter counts something > different. Any hard-to-optimize loop risks becoming wrong on the other side (e.g. if something stalls the pipeline, a newer chip with longer pipeline will use more CPU cycles). Turbo boost could also contribute to lowering the number of cycles; a boosted processor has ref cpu cycles that are _longer_ than the regular cycles (thus they count in smaller numbers). Maybe that's why "core cycles" didn't go below N. The real result was something like 0.8*N (780-830000). I used 0.1*N because it is used for the "ref cpu cycles" gp counter, which is not the same but similar. Should I change it to 0.5*N or so? Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html