Re: [Xen-devel] [PATCH V4] Switch from select() to poll() in xenconsoled's IO loop

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-01-07 at 15:06 +0000, Mats Petersson wrote:
> On 07/01/13 15:01, Wei Liu wrote:
> > On Mon, 2013-01-07 at 14:41 +0000, Mats Petersson wrote:
> >
> >>>                           return;
> >>> @@ -982,11 +1024,7 @@ void handle_io(void)
> >>>                   /* Re-calculate any event counter allowances & unblock
> >>>                      domains with new allowance */
> >>>                   for (d = dom_head; d; d = d->next) {
> >>> -                       /* Add 5ms of fuzz since select() often returns
> >>> -                          a couple of ms sooner than requested. Without
> >>> -                          the fuzz we typically do an extra spin in 
> >>> select()
> >>> -                          with a 1/2 ms timeout every other iteration */
> >>> -                       if ((now+5) > d->next_period) {
> >>> +                       if (now > d->next_period) {
> >> Is poll more accurate than select? I would have thought that they were
> >> based on the same timing, and thus equally "fuzzy"?
> > Is there any actual proof that the fuzz is needed? Specs of both
> > select() and poll() don't seem to mention this behaviour at all.
> That's a good question. I don't know. The tricky part with this sort of 
> thing is that it may well depend on configurations, hardware 
> differences, etc, so you may find that it works just fine on your 
> test-box, but some big customer with Another-brand Co's servers don't 
> work, because there is some subtle difference in hardware. Or it stops 
> working if you have more than X number of CPU's. If you are convinced 
> it's fine as it is, then by all means. I'm just thinking that it 
> probably wasn't put there "by accident".

There's certainly an argument for removing it in a separate changeset
though in case it does cause issues.

Ian


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