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Date: Fri, 7 Apr 2017 11:43:49 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: [RFC][PATCH] spin loop arch primitives for busy waiting
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On Thu, Apr 06, 2017 at 10:31:46AM -0700, Linus Torvalds wrote:
> But maybe "monitor" is really cheap. I suspect it's microcoded,
> though, which implies "no".

On my IVB-EP (will also try on something newer):

MONITOR	~332 cycles
MWAIT	~224 cycles (C0, explicitly invalidated MONITOR)

So yes, expensive.