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Re: [Qemu-devel] [PATCH 0/3] New sigaltstack method for coroutine


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 0/3] New sigaltstack method for coroutine
Date: Tue, 14 Feb 2012 12:17:55 +0000

On Tue, Feb 14, 2012 at 11:38 AM, Alex Barcelo <address@hidden> wrote:
> On Tue, Feb 14, 2012 at 09:33, Stefan Hajnoczi <address@hidden> wrote:
>> On Mon, Feb 13, 2012 at 04:11:15PM +0100, Alex Barcelo wrote:
>>> This new implementation... well, it seems to work (I have done an
>>> ubuntu installation with a cdrom and a qcow drive, which seems to use
>>> quite a lot of coroutines). Of course I have done the coroutine-test
>>> and it was OK. But... I wasn't confident enough to propose it as a
>>> "mature alternative". And I don't have any performance benchmark,
>>> which would be interesting. So, I thought that the better option would
>>> be to send this patch to the developers as an alternative to ucontext.
>>
>> As a starting point, I suggest looking at
>> test-coroutine.c:perf_lifecycle().  It's a simple create-and-then-enter
>> benchmark which measures the latency of doing this.  I expect you will
>> find performance is identical to the ucontext version because the
>> coroutine should be pooled and created using sigaltstack only once.
>>
>> The interesting thing would be to benchmark ucontext coroutine creation
>> against sigaltstack.  Even then it may not matter much as long as pooled
>> coroutines are used most of the time.
>
> Didn't see the performance mode for test-coroutine. Now a benchmark
> test it's easy (it's half-done). The lifecycle is not a good
> benchmark, because sigaltstack is only called once. (As you said, the
> timing change in less than 1%).
>
> I thought that it would be interesting to add a performance test for
> nesting (which can be coroutine creation intensive). So I did it. I
> will send as a patch, is simple but it works for this.
>
> The preliminary results are:
> ucontext (traditional) method:
> MSG: Nesting 1000000 iterations of 100000 depth each: 0.452988 s
>
> sigaltstack (new) method:
> MSG: Nesting 1000000 iterations of 100000 depth each: 0.689649 s

Plase run the tests with more iterations.  The execution time should
be several seconds to reduce any scheduler impact or other hickups.  I
suggest scaling iterations up to around 10 seconds.

> The sigaltstack is worse (well, it doesn't surprise me, it's more
> complicated and does more jumps and is a code flow more erratic). But
> a loss in efficiency in coroutines should not be important (how many
> coroutines are created in a typical qemu-system execution? I'm
> thinking "one"). Also as you said ;) pooled coroutines are used most
> of the time, in real qemu-system execution.

No, a lot of coroutines are created - each parallel disk I/O request
involves a coroutine.  Coroutines are also being used in other
subsystems (e.g. virtfs).

Hopefully the number active coroutines is still <100 but it's definitely >1.

Stefan



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