lkml.org 
[lkml]   [2019]   [Sep]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics
From
Date


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


\
 
 \ /
  Last update: 2019-09-09 15:41    [W:0.110 / U:0.032 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site