RE: [Xen-devel] Re: [PATCH] Align periodic vpts

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> > + * CAUTION:
> > + * While vlapic timer ticking too close to the pit.  We 
> saw a userspace
> > + * application getting the wrong answer because long CPU 
> bound sequences
> > + * appeared to run with zero CPU time. This only showed up 
> with old Linux
> > + * kernels (IIRC, it was with Red Hat 3 U8). So this 
> option may cause a
> > + * regression in this case.
> > + */
> > +static int opt_align_periodic_vpt = 0;
> > +boolean_param("align_periodic_vpt", opt_align_periodic_vpt);
> > +
> 
> Presumably there are common cases where not aligning vlapic too has
> significant power overheads? Personally I'm not sure I care 
> too much about a
> minor regression on RH3, if this patch is worthwhile at all I think it
> should be always on and at most have a domain config option. 
> I think a boot
> option will never ever be used.

Given the wide variety of guests, and clocksource defaults/choices
in those guests, I'm leery about this change, especially turning it
on by default.  The consequences of guest clocks losing time or gaining
time or appearing to go backwards are significant and the potential
problems go well beyond "a minor regression on RH3" and IMHO are
much more impactful for customers than saving a watt or two.

It is difficult in a simple test environment to reproduce the
problem unless you know what you are looking for.
Virtual Iron had a rather extensive set of test cases and I'd
like to see those run before this is turned on by default.

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