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Date:      Thu, 29 Jan 2015 22:51:22 -0500
From:      Ryan Stone <rysto32@gmail.com>
To:        John Baldwin <jhb@freebsd.org>
Cc:        "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org>
Subject:   Re: svn commit: r277900 - head/sys/x86/x86
Message-ID:  <CAFMmRNxaartbZZ875F_YF01L7nQ%2BLKcd%2BUnD0RJdbJ=m0x5jbQ@mail.gmail.com>
In-Reply-To: <8728880.CZu0UL854D@ralph.baldwin.cx>
References:  <201501292041.t0TKfhAJ029429@svn.freebsd.org> <8728880.CZu0UL854D@ralph.baldwin.cx>

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On Thu, Jan 29, 2015 at 3:45 PM, John Baldwin <jhb@freebsd.org> wrote:
> Also, note that we always use the TSC as the cpu ticker, even if it is not the
> timecounter.  Thus, if C2+ is enabled on the systems, the kernel idle threads
> will have an artificially low runtime since their idle time is not properly
> accounted for.  This does not affect any other threads as only the idle
> threads "run" while the CPU is in C2+ with the TSC effectively paused.  The
> new default means that the idle threads will now have the proper runtime,
> however.

However in a VM environment this means that runtimes for all threads
can be very wrong (regardless of whether we enter C2 or lower or not).
I have a coworker looking at using kvmclock in preference to TSC for
the cpu ticker when the kvmclock is available.



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