linux-kernel - Re: [PATCH] x86: Add clflush before monitor for Intel 7400 series

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Message-ID: <48F8F1DB.9030202@zytor.com>
Date:	Fri, 17 Oct 2008 13:13:15 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Add clflush before monitor for Intel 7400 series

Pallipadi, Venkatesh wrote:
> 
> hpa,
> 
> Do you still have reservations about this being expensive.
> Note that this is only done when a CPU is about to go idle and
> The cost of clflush itself will be minimal compared to idle
> entry + idle exit latency.
> 

I guess I'm a bit confused about the tradeoff of CLFLUSH versus simply 
disabling MWAIT.  This is a relatively recent processor and so 
optimizing matters (if this was a P4 I would be more worried about what 
has least impact on the kernel as a whole.)

Entry and exit latency do matter (specifically, exit latency matters for 
longer waits and the combined entry+exit latency matters for short waits.)

On the other hand, perhaps what we need to do is to get the fix in and 
worry about performance later.

	-hpa
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