Messages in this thread | | | Date | Thu, 13 Nov 2003 11:54:46 +1100 | From | Nick Piggin <> | Subject | Re: So, Poll is not scalable... what to do? |
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Davide Libenzi wrote:
>On 12 Nov 2003, bill davidsen wrote: > > >>In article <20031112053207.GA9634@alpha.home.local>, >>Willy Tarreau <willy@w.ods.org> wrote: >>| On Tue, Nov 11, 2003 at 05:52:42PM -0600, kirk bae wrote: >>| > If poll is not scalable, which method should I use when writing >>| > multithreaded socket server? >>| >>| Honnestly, if you're using threads (I mean lots of threads, such as one >>| per connection), I don't think that poll performance will be your worst >>| ennemy. The first thing to do is to handle the task switching yourself >>| either with a publicly available coroutine library or with one of your own. >> >>It's not clear that with 2.6 this is necessary or desirable. I'll let >>someone who worked on the new thread and/or futex development say more >>if they will, but I'm reasonable convinced that in most cases the kernel >>will do it better. >> > >Pros & Cons: > >*) Coroutines cost is basically its stack (8-16Kb). Threads there's a >little bit more under the hood > >*) No locks at all with coroutines > >*) Coroutine context switch time was about 20 times faster last time I >tried. I used this: >
According to lmbench, 2.4 does a context switch in 0.67us (on one type of uniprocessor cpu - the machines at osdl). 2.6.0-test9 manages 1.17us (174% longer). I have some patches that brings this to 0.80us (119%). This is with 2 active tasks, so O(1) doesn't get to show its advantage.
If you remove the realtime priority array (disable RT scheduling support) you could probably drop this figure below 2.4's. I should get some numbers
Perhaps if there is demand I could make RT scheduling a config option in my patchset. I think Andrea does something fancy like that. I'll have to take a look at his code.
That said, I'd be inclined to think an application that it context switch bound is broken by design. Although maybe there are some special cases.
2.4: http://khack.osdl.org/stp/282982/results/summary_report 2.6: http://khack.osdl.org/stp/282354/results/summary_report np: http://khack.osdl.org/stp/283054/results/summary_report
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