Messages in this thread | | | Subject | Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics | From | "Liang, Kan" <> | Date | Mon, 9 Sep 2019 09:40:26 -0400 |
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On 8/31/2019 5:19 AM, Peter Zijlstra wrote: >>> Then there is no mucking about with that odd counter/metrics msr pair >>> reset nonsense. Becuase that really stinks. >> You have to write them to reset the internal counters. > But not for ever read, only on METRIC_OVF.
The precision are lost if the counters were running much longer than the measured delta. We have to reset the counters after every read. For example, the worst case is: The previous reading was rounded up and the current reading is rounded down. The error of METRICS is 1/256 - 1/SLOTS, which is related to SLOTS. The error for each Topdown event is (1/256 - 1/SLOTS) * SLOTS. With the SLOTS increasing, the precision will get worse and worse. Therefor, we have to reset the counters periodically.
Thanks, Kan
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