Re: status of Preemptible Kernel 2.6.7

[Timothy Miller <miller@techsource.com>]

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.