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Subject: RE: HT and idle = poll
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Date: Thu, 6 Mar 2003 13:15:43 -0800
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Thread-Topic: HT and idle = poll
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From: "Nakajima, Jun" <jun.nakajima@intel.com>
To: "Linus Torvalds" <torvalds@transmeta.com>,
       "Alan Cox" <alan@lxorguk.ukuu.org.uk>
Cc: "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
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Linus,

That's correct. Basically mwait is similar to hlt, but you can avoid IPI to wake up the processor waiting. A write to the address specified by monitor wakes up the processor, unlike hlt.

So our plan is to use monitor/mwait in the idle loop, for example, in the kernel to lower the latency.

Jun

> -----Original Message-----
> From: Linus Torvalds [mailto:torvalds@transmeta.com]
> Sent: Thursday, March 06, 2003 12:09 PM
> To: Alan Cox
> Cc: Linux Kernel Mailing List
> Subject: Re: HT and idle = poll
> 
> 
> On 6 Mar 2003, Alan Cox wrote:
> > On Thu, 2003-03-06 at 19:30, Linus Torvalds wrote:
> > > >So, don't use idle=poll with HT when you know your workload has idle
> time!  I
> > > >have not tried oprofile, but it stands to reason that this would be a
> >
> > idle=poll probably needs to be doing "rep nop" in a tight loop.
> 
> We already do that. It's not enough. The HT thing will still steal cycles
> continually, since the "rep nop" is really only equivalent to a
> "sched_yield()".
> 
> Think of "rep nop" as yielding, and "mwait" as a true wait.
> 
> (I don't actually have any real information on "mwait", so I may be wrong
> about the details on the new instructions. They looked obvious enough,
> though).
> 
> 		Linus
> 
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