amd64/152405: wrong user time is reported when CPU frequency
KOIE Hidetaka ( 鯉江英隆＠数理技研 )
koie at suri.co.jp
Thu Nov 25 04:00:20 UTC 2010
The following reply was made to PR amd64/152405; it has been noted by GNATS.
From: KOIE Hidetaka (=?iso-2022-jp?B?GyRCOHE5PjFRTjQhdz90TX01OzgmGyhC?=)
<koie at suri.co.jp>
To: avg at freebsd.org
Cc: bug-followup at freebsd.org
Subject: Re: amd64/152405: wrong user time is reported when CPU frequency
Date: Thu, 25 Nov 2010 12:56:40 +0900 (JST)
Message-Id: <4CED1EEC.5010107 at freebsd.org>
Date: Wed, 24 Nov 2010 16:19:24 +0200
From: Andriy Gapon <avg at freebsd.org>
Subject: Re: amd64/152405: wrong user time is reported when CPU f..
| on 24/11/2010 15:54 Andriy Gapon said the following:
| > Anyway, it seems that I missed the real cause of this problem and the excellent
| > explanation by Bruce.
| Could you please try the following (work-in-progress) patch?
on the patched kernel, time command reports correct time!
% sysctl kern.eventtimer.timer
% sudo sysctl dev.cpu.0.freq=1800
dev.cpu.0.freq: 1800 -> 1800
% time ./loop
4.835u 0.007s 0:04.99 96.7% 5+2785k 0+0io 0pf+0w
% sudo sysctl dev.cpu.0.freq=1000
dev.cpu.0.freq: 1800 -> 1000
% time ./loop
8.754u 0.000s 0:09.12 95.9% 5+2777k 0+0io 0pf+0w
| I expect that it should improve accuracy for your testcase, but may penalize
| system performance to certain degree.
| As mav@ has said, performance, accuracy, powerstate management - pick any two of
| the three. OTOH, I am not sure by how much the performance could be affected.
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