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Date: Mon, 03 Nov 2008 13:15:21 +0200
From: Krassimir Slavchev <krassi@bulinfo.net>
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To: Julian Elischer <julian@elischer.org>
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Cc: freebsd-performance@freebsd.org, Sam Xia <xiazhongqi@huawei.com>
Subject: Re: 1. thread switching time? (Krassimir Slavchev)
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Julian Elischer wrote:
> Krassimir Slavchev wrote:
> 
>>
>> Also, I am able to set HZ=100000 but the thread switching time is still
>> ~1ms.
> 
> You must be talking about the time that a thread can run before it is
> pushed out by other threads?  I know I've seem many x 10,000 context
> switches in some cases, i.e. look at very high interrupt rates
> etc. so I guess I'm not sure what you are measuring..
> 
> at one time the scheduling quantum was independent of Hz.
> I am not sure how true that is now..
> 

The scheduling quantum is independent of Hz for Hz > 1000.

usr/src/sys/kern_tc.c:
...
	if (hz > 1000)
		tc_tick = (hz + 500) / 1000;
	else
		tc_tick = 1;
...

I am not sure whether setting Hz > 1000 impact on performance or not,
e.g using polling(4).
Removing this limitation and setting kern.hz=10000 give 100usec quantum.

Thanks for the point.

Best Regards
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