Message-ID: <40D9DA4A.3070700@techsource.com>
Date: Wed, 23 Jun 2004 15:30:18 -0400
From: Timothy Miller <miller@techsource.com>
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To: Robert Love <rml@ximian.com>
CC: Marcus Hartig <m.f.h@web.de>, linux-kernel@vger.kernel.org
Subject: Re: status of Preemptible Kernel 2.6.7
References: <40D9B20A.4070409@web.de>  <40D9C48C.4060004@techsource.com> <1088017171.14159.2.camel@betsy>
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Robert Love wrote:
> On Wed, 2004-06-23 at 13:57 -0400, Timothy Miller wrote:
> 
> 
>>I vaguely recall someone recently talking about eliminating preempt by 
>>improving low-latency.  See, if everything were ideal, we wouldn't need 
>>preempt, because all drivers would yield the CPU at appropriate times. 
> 
> 
> If everything held locks for only sane periods of time, we would not
> need gross explicit yielding all over the place.
> 
> To answer Marcus's question: go for it and use it.


I wasn't talking about locks.  I was talking about kernel functions 
taking long periods of time, cases where preempt has been useful to 
reduce kernel latency.

Holding locks for extended periods is something else entirely.

I presume there are sane cases where a kernel function will need to 
execute for a "long time", like when doing PIO disk access or COW, etc. 
  It would be good to have a way to limit the impact of those functions 
in terms of user-perceived latency, just as preempt has done, but 
without preempt.

At least, I thought that was the idea.

Now, the thing is, if you have explicit cooperative yields, then a slow 
CPU might not yield often enough, and a fast CPU would yield too often. 
    Preempt has the advantage of using real time so that CPUs can 
maximize throughput without affecting latency.


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