Messages in this thread | | | Date | Mon, 21 Jan 2002 09:50:51 -0700 | From | yodaiken@fsmlabs ... | Subject | Re: [2.4.17/18pre] VM and swap - it's really unusable |
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On Mon, Jan 21, 2002 at 05:48:30PM +0100, Daniel Phillips wrote: > On January 21, 2002 05:06 pm, yodaiken@fsmlabs.com wrote: > > On Mon, Jan 21, 2002 at 05:05:01PM +0100, Daniel Phillips wrote: > > > > I think of "benefit", perhaps naiively, in terms of something that can > > > > be measured or demonstrated rather than just announced. > > > > > > But you see why asap scheduling improves latency/throughput *in theory*, > > > > Nope. And I don't even see a relationship between preemption and asap I/O > > schedulding. What make you think that I/O threads won't be preempted by > > other threads? > > Consider a thread reading from disk in such a way that readahead is no help, > i.e., perhaps the disk is fragmented. At each step the IO thread schedules a > read and sleeps until the read completes, then schedules the next one. At > the same time there is a hog in the kernel, or perhaps there is > competition from other tasks using the kernel. In any event, it will > frequently transpire that at the time the disk IO completes there is somebody > in the kernel. Without preemption the IO thread has to wait until the kernel > hog blocks, hits a scheduling point or exits the kernel.
So your claim is that: Preemption improves latency when there are both kernel cpu bound tasks and tasks that are I/O bound with very low cache hit rates?
Is that it?
Can you give me an example of a CPU bound task that runs mostly in kernel? Doesn't that seem like a kernel bug?
> Naturally I constructed this case to show the effect most clearly. There are
How about a plausible case?
> many possible variations on the above scenario. It does seem to explain the > latency/throughput improvements that have been reported in practice.
I still keep missing these reports. Can you help me here? (Obviously "my laptop seems more effervescent" is not what I'm looking for.)
-- --------------------------------------------------------- Victor Yodaiken Finite State Machine Labs: The RTLinux Company. www.fsmlabs.com www.rtlinux.com
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