Date: Fri, 31 Jan 2003 10:55:40 -0800 From: Peter Wemm <peter@wemm.org> To: Nate Lawson <nate@root.org> Cc: John Baldwin <jhb@FreeBSD.org>, cvs-committers@FreeBSD.ORG, cvs-all@FreeBSD.ORG Subject: Re: cvs commit: src/sys/i386/i386 mp_machdep.c Message-ID: <20030131185540.84A122A8A1@canning.wemm.org> In-Reply-To: <Pine.BSF.4.21.0301311023040.96929-100000@root.org>
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Nate Lawson wrote:
> On Fri, 31 Jan 2003, John Baldwin wrote:
> > On 30-Jan-2003 Matthew Dillon wrote:
> > > Ouch. That's rather counter-intuitive since idle_hlt usually
> > > degrades performance, but I see how it would improve performance on
> > > an HTT box.
> >
> > Well, idle_hlt can only really hurt if you are doing compute bound stuff.
> > You get at least 128 clock interrupts per second from the RTC that get
> > IPI'd to all the other CPUs, so no CPU would errantly stay halted for more
> > than 1/128th of a second with idle_hlt on anyways. As Peter has mentioned,
> > the idle_hlt thing shows some rather impressive thermal benefits, and I
> > imagine it can provide power savings of some sort. I would argue that
> > being fully compute bound is not the common case and that we should probabl
y
> > default to having it on in general.
>
> Why not check CPU utilization and dynamically en/disable HLT?
I was thinking of doing an idle cpu mask and adding an IPI to do a wakeup.
That way we can just kill the problem and be done with it.
Have you considered promisciously turning on the idle_hlt flag by the
HTT detection code? ie: if booting with >1 logical cores, then turn
it on at boot.
Also, some folks measured a slight increase in performance by turning on
idle_hlt, which presumably was due to less cache line thrashing. This will
obviously have different effects on different cpu architectures (p3, p4,
athlon etc).
Anyway, I really like it, because I've been able to stop my 6-month cpu fan
replacement cycle that has been as regular as clockwork for the last 5 years.
PPro fans are getting harder to find these days. :-] The last set have lasted
just over a year and have no signs of wearing out. (I bought a small box of
them, figuring I was using them so fast and they were hard to find....)
Cheers,
-Peter
--
Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5
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