RE: [Xen-devel] [PATCH] TSC scaling for live migration between platforms with different TSC frequecies

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> > Just checking... this is in 10 seconds and each processor is
> > "ticking" (and possibly a system-wide timer tick as well),
> > so this is ~350 rdtsc/sec/processor, correct?
> 
> No. That's CPU0 only ('cpu == 0'). Solaris only has one system-wide
> timer tick. This is mstate accounting: every kernel/user 
> boundary, every
> interrupt, etc. incurs at least one TSC read. (And of course 
> the machine
> is idle.)

Wow.  (and I repeat for emphasis) Wow.

Even when restricted to physical hardware, using the TSC
for such purposes seems ill-advised.  In a virtual data
center, the data will be often useless.

Is mstate accounting used for anything other than providing
interesting performance data if one cares to look at it?
Does mstate accounting ignore negative values for delta TSC?

Well, even a few thousand RDTSC/second is not too bad
if RDTSC emulation can be brought in under a couple thousand
cycles.  (I don't know that it can, but I also don't know
that it can't.)

Dan

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