Date: Fri, 18 Mar 2016 11:58:19 -0700 From: Stanislav Sedov <stas@FreeBSD.org> To: Ryan Stone <rysto32@gmail.com> 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> Subject: Re: Peformance issues with r278325 Message-ID: <59BA610B-3754-43D6-839D-84FEE8F13CF7@FreeBSD.org> In-Reply-To: <CAFMmRNz1PnzHDstZ4sgMs7aTgA_c0ydkxaFCi%2B8We6k3GG8wGw@mail.gmail.com> References: <FA50A68E-7F3D-4361-8A8A-EB7F97EF3D00@FreeBSD.org> <8403291.NqUNo0Qq5W@ralph.baldwin.cx> <CAJ-VmonUdHRJ0jcFphqXi1w2PwbDycLG190Yd5z8JQWGxW_1iQ@mail.gmail.com> <3277812.DVsZx4uMun@ralph.baldwin.cx> <CAFMmRNz1PnzHDstZ4sgMs7aTgA_c0ydkxaFCi%2B8We6k3GG8wGw@mail.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
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