Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics

Peter Zijlstra
2019-08-31T09:29:34Z

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' :/