Re: So, Poll is not scalable... what to do?

Nick Piggin
2003-11-13T00:54:54Z

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