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Subject: Re: Peformance issues with r278325
From: Stanislav Sedov <stas@FreeBSD.org>
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Date: Fri, 18 Mar 2016 11:58:19 -0700
Cc: John Baldwin <jhb@freebsd.org>, Adrian Chadd <adrian.chadd@gmail.com>,
 Konstantin Belousov <kostikbel@gmail.com>,
 "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>
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To: Ryan Stone <rysto32@gmail.com>
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> On Mar 18, 2016, at 11:49 AM, Ryan Stone <rysto32@gmail.com> wrote:
>=20
> On Fri, Mar 18, 2016 at 1:37 PM, John Baldwin <jhb@freebsd.org> wrote:
> I think I'll likely just convert it to use a direct
> TSC delay loop always in HEAD (assuming that verifies ok in testing as =
well).
>=20
> Couldn't that work incorrectly on VM guests?  The tsc is not =
guaranteed to be monotonic in that environment.
>=20

Another concern is SMP systems where the clock is not synchronized.  SMP =
TSC requires
a complicated setup procedure on the system boot which is not followed =
properly by all
vendors, and I have seen some recent systems with SMP TSC skew.

I'm afraid that using TSC in this code will make FreeBSD unusable on =
such (arguably buggy)
systems.=20

--
Stanislav Sedov
ST4096-RIPE