From mboxrd@z Thu Jan  1 00:00:00 1970
From: John Levon <levon@movementarian.org>
Subject: Re: [PATCH] TSC scaling for live migration between
	platforms with different TSC frequecies
Date: Thu, 18 Jun 2009 16:45:51 -0400
Message-ID: <20090618204551.GA25732@movementarian.org>
References: <20090618164535.GA30082@movementarian.org>
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To: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Ian Pratt <Ian.Pratt@eu.citrix.com>, xen-devel@lists.xensource.com, Keir Fraser <Keir.Fraser@eu.citrix.com>, "Zhang,
	Xiantao" <xiantao.zhang@intel.com>
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On Thu, Jun 18, 2009 at 01:27:23PM -0700, Dan Magenheimer wrote:

> > Certainly Solaris relies on the TSC for time-keeping, and uses it very
> > heavily. To the extent that I doubt it's even feasible to migrate to a
> > machine where scaling needs to be done, and such a migration should be
> > refused, since it would essentially kill the guest.
> 
> Hmmm... any numbers?  Certainly Solaris isn't reading TSC much

# dtrace -n 'fbt::tsc_gethrtime:entry /cpu == 0/ { @ = sum(1); }' -c "sleep 10"
dtrace: description 'fbt::tsc_gethrtime:entry ' matched 1 probe
dtrace: pid 29708 has exited

            27798

This is on a basically idle 8-way system. (The other CPUs are less busy.)

http://blogs.sun.com/eschrock/entry/microstate_accounting_in_solaris_10

> that data centers running Solaris guests must segregate sets of
> their machines by clock rate and disallow migrations
> between the sets?

Certainly for Solaris HVM that has to be the case until we make it use PV time
(presuming that is safe, which I'm not sure offhand).

> THAT expensive on x86?  (If max(TSC/sec/processor)~=1000 and
> cycles/emulation~=5000, total degradation would be
> less than 1%.  (Sounds high, but if the alternative is

At the end of the day, though, only testing will tell us for sure.

regards
john