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 15:01 +0000, 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.

I agree that it seems pretty dubious but it might be interesting to see
what strace shows, specifically if it shows this extra spin every other
iteration.

The fuzz was introduced in 16257:955ee4fa1345 "Rate-limit activity
caused by each domU." but the commit log doesn't make any reference to
the reason for it.

It may just have been a kernel bug at around the time that patch was
authored, but google doesn't seem to show any evidence of such a bug
ever being widespread (i.e. I don't find any references to it).

Ian.


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