Messages in this thread | | | Date | Sat, 31 Aug 2019 11:29:21 +0200 | From | Peter Zijlstra <> | Subject | Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics |
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On Sat, Aug 31, 2019 at 02:13:05AM -0700, Stephane Eranian wrote:
> With PERF_METRICS, the delta is always since previous read. If you read > frequently enough you do not lose precision.
You always loose precision, the whole fraction of 255 looses the fractional part; consider:
255 * 1 / 8 31.87500000000000000000 255 * 7 / 8 223.12500000000000000000
31 * 8 / 255 .97254901960784313725 223 * 8 / 255 6.99607843137254901960
Now, we can make reconstruction use normal 'round-nearest' and then we'd get 1 and 7 back, but there's always cases where you loose things.
Also, with 255 being an odd number, round-nearest is actually 'hard' :/
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