linux-kernel - Re: why kernel implement "udelay" by cpu instructions?

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Date:	Wed, 4 Nov 2009 00:36:48 -0500
From:	Bryan Donlan <bdonlan@...il.com>
To:	Rajat Jain <Rajat.Jain@...ogain.com>
Cc:	loody <miloody@...il.com>, Rik van Riel <riel@...hat.com>,
	linux-kernel@...r.kernel.org,
	Kernel Newbies <kernelnewbies@...linux.org>
Subject: Re: why kernel implement "udelay" by cpu instructions?

On Wed, Nov 4, 2009 at 12:01 AM, Rajat Jain <Rajat.Jain@...ogain.com> wrote:
> Hi,
>
>> I find something interesting; kernel has msleep, but it
>> doesn't have usleep.
>> Does that mean the minimum time kernel can react is msecond
>> instead of usecond?
>> so if  users want to count useconds, they have to do the busy waiting,
>> execute some looping assembly instructions?
>
> You are roughly right. If you don't want to busy loop (udelay / mdelay), then you will have to sleep. The granularity of this sleep depends on how frequently the timer interrupt ticks (HZ). Thus if HZ is 1000, then you cannot sleep for a period less than 1 msec.

I thought hrtimers allow higher-precision wakeups these days?
Of course, if you only want to sleep for a few microseconds, the
context switch might take longer than you want to sleep...
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