Re: [PATCH 5/6] futex: handle timeout inside adaptive lock spin

From: Darren Hart
Date: Wed Apr 07 2010 - 19:15:35 EST

Gregory Haskins wrote:
>>>> On 4/7/2010 at 01:31 PM, in message <4BBCC174.7020409@xxxxxxxxxx>, Darren Hart
> <dvhltc@xxxxxxxxxx> wrote:
>> Thomas Gleixner wrote:
>>> On Mon, 5 Apr 2010, Darren Hart wrote:
>>>> Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>
>>>> + if (timeout) {
>>>> + now = ktime_get();
>>> Hmm. Calling that in every iteration might hurt especially on non
>>> TSC systems, but well...
>> I haven't come across a better alternative since arming the timer before
>> setting TASK_INTERRUPTIBLE isn't appropriate.
> Hey Darren,
> I remember we tried something similar in early versions of the
> adaptive locks and this was definitely bad. :(
> It ended up putting so much contention on the xtime_lock (IIRC) that
> it resulted in adaptive locks hurting overall performance verses not
> using adaptive at all. Alternative mechanisms employed a hybrid where
> the inner loops used a pseudo calibrated counter loop, and the outer
> loop checks periodically against a real clock. It all plays into "you
> are burning CPU cycles anyway, so might as well put them to use"
> theory. Hacky, yes, but it did relieve the pressure on the time
> subsystem locks and freed up a _lot_ of performance. Without this,
> the concept of timeouts+adaptive was unworkable. I think Steven
> ultimately rejected the timeout related patches outright when he
> merged adaptive to -rt, but I think Sven pulled them into SLERT if you
> would like a potential code reference to a working solution.

Hi Greg,

Thanks for the info! I haven't tested with timeouts yet as I'm still struggling to get decent performance out of just plain old adaptive right now. I'll keep that in mind when I get to that, and yeah, if the plan is to burn CPU cycles, might as well do something constructive :-)

I do feel the timeouts are a necessary feature. Interruptibility may be as well, but I'm going to ignore it for the time being...

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