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Date: Sun, 14 Jun 2009 13:39:37 +0100
From: Kris Kennaway <kris@FreeBSD.org>
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To: John Baldwin <jhb@freebsd.org>
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Cc: acpi <acpi@freebsd.org>, freebsd-hackers@freebsd.org,
	freebsd-acpi@freebsd.org, Pieter de Goeje <pieter@degoeje.nl>,
	freebsd-performance@freebsd.org
Subject: Re: ACPI-fast default timecounter, but HPET 83% faster
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John Baldwin wrote:
> On Sunday 26 April 2009 10:27:42 pm Garrett Cooper wrote:
>> I'm seeing similar results.
>>
>> [root@orangebox /usr/home/gcooper]# dmesg | grep 'Timecounter "'
>> Timecounter "i8254" frequency 1193182 Hz quality 0
>> Timecounter "ACPI-fast" frequency 3579545 Hz quality 1000
>> Timecounter "HPET" frequency 14318180 Hz quality 900
>> [root@orangebox /usr/home/gcooper]# ./cgt
>> 1369355
>> [root@orangebox /usr/home/gcooper]# sysctl
>> kern.timecounter.hardware="ACPI-fast"
>> kern.timecounter.hardware: HPET -> ACPI-fast
>> [root@orangebox /usr/home/gcooper]# ./cgt
>> 772289
>>
>> Why's the default ACPI-fast? For power-saving functionality or because
>> of the `quality' factor? What is the criteria that determines the
>> `quality' of a clock as what's being reported above (I know what
>> determines the quality of a clock visually from a oscilloscope =])?
> 
> I suspect that the quality of the HPET driver is lower simply because no one
> had measured it previously and HPET is newer and less "proven".
> 

 From memory, HPET was massively slower on some of the AMD test hardware 
I was using.  There was a thread about it on one of the mailing lists, 
but I can't find it right now.

Kris