[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Xen-devel] Re: linux c/s 561



Someone reported they could spin over twenty times before a soon-to-occur
jiffy 'tick' (specifically, the 'jiffies+1' tick) actually occurred. Which
makes sense since the 'slop' in Xen's timer_softirq_action() is 50us and the
cost of block-plus-wakeup-on-VIRQ_TIMER is about a couple of microseconds.
So that's over twenty times setting a timeout less than 50us in the future,
blocking, immediately waking up in the timer ISR, doing no work, setting a
timeout... etc.

So the patch avoids this little corner case. It's not a major issue, and the
side effect is that you may wait a little longer to do timer work.

 -- Keir

On 2/6/08 08:46, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> Keir,
> 
> this change looks a little odd to me - the code as it was before already
> set the timeout to jiffies + 1 as the lowest possible value. So I don't
> understand how you could have observed any spinning over more than
> really short periods of time (where you happen to just see jiffies
> increment while setting the timeout).
> 
> I'd therefore appreciate any insight to the background of that change.
> 
> Thanks, Jan
> 



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.