HZ=1000 in Fedora kernels

[Brendan Jones]

On 15/03/13 06:46, Prarit Bhargava wrote:
>
>
> On 03/14/2013 02:43 PM, Zach Brown wrote:
>>> IIRC, the scheduler timeslice isn't impacted by HZ.
>>
>> Yeah. As Dave was pointing out, a lot of the timeout aliasing problems
>> with HZ have been fixed by moving implementations of waits from the HZ
>> granular interfaces to hrtimers.  f.e.
>>
>>      commit 8ff3e8e85fa6c312051134b3953e397feb639f51
>>      Author: Arjan van de Ven <arjan at linux.intel.com>
>>      Date:   Sun Aug 31 08:26:40 2008 -0700
>>
>>          select: switch select() and poll() over to hrtimers
>>
>> has at its core:
>>
>> -               __timeout = schedule_timeout(__timeout);
>>
>> +               if (!schedule_hrtimeout(to, HRTIMER_MODE_ABS))
>>
>> A slightly different and disappointing result of dropping HZ is
>> increasing the duration of waits in code that is still using
>> schedule_timeout(1) for a short timeout.
>>
>> btrfs has a bunch of these that are trying to wait for more work to
>> accumulate before carrying on.  If you drop HZ you'll be adding 4ms (or
>> 10ms) delays to a few paths.
>>
>> jbd has similar code that is sensitive to jiffies, but it's a little
>> more involved because it's measuring journal commit times rather than
>> using a dumb single jiffie timeout.
>
> IOW .. don't modify HZ=1000 yet ;).  Durnit ...
>

1000HZ has long been recommended for accurate MIDI playback,

regards,

Brendan

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