linux-kernel - Re: [PATCH 1/2] cpuidle: menu: avoid expensive square root computation

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Message-ID: <56C3AEB2.4000007@intel.com>
Date:	Wed, 17 Feb 2016 00:20:18 +0100
From:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Andi Kleen <andi@...stfloor.org>, Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, akpm@...ux-foundation.org,
	lenb@...nel.org, Eric Dumazet <eric.dumazet@...il.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cpuidle: menu: avoid expensive square root
 computation

On 2/16/2016 8:19 PM, Rasmus Villemoes wrote:
> Computing the integer square root is a rather expensive operation, at
> least compared to doing a 64x64 -> 64 multiply (avg*avg) and, on 64
> bit platforms, doing an extra comparison to a constant (variance <=
> U64_MAX/36).
>
> On 64 bit platforms, this does mean that we add a restriction on the
> range of the variance where we end up using the estimate (since
> previously the stddev <= ULONG_MAX was a tautology), but on the other
> hand, we extend the range quite substantially on 32 bit platforms - in
> both cases, we now allow standard deviations up to 715 seconds, which
> is for example guaranteed if all observations are less than 1430
> seconds.
>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>

Both patches look good to me,  so I'm going to queue them up for 4.6.

If anyone has any issues with that, please let me know.

Thanks,
Rafael