linux-kernel - Re: [PATCH] lib/int_sqrt.c: Optimize square root function

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Message-ID: <CA+HOOsgiV_CG5tWEC91-h0wcfP5oZJ02ztOCr+=WjM6wHjVufA@mail.gmail.com>
Date:	Sun, 8 Feb 2015 21:09:27 +0530
From:	Anshul Garg <aksgarg1989@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <dave@...olabs.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"anshul.g@...sung.com" <anshul.g@...sung.com>
Subject: Re: [PATCH] lib/int_sqrt.c: Optimize square root function

Dear Mr. linus,

Thanks for quick replies.

Yes performance numbers are not conclusive enough.
So its better to discard this patch as of now.

I will try to explore more in this area.


Thanks & regards
Anshul Garg



On Fri, Feb 6, 2015 at 1:07 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Feb 5, 2015 at 10:43 AM, Anshul Garg <aksgarg1989@...il.com> wrote:
>>
>> NOTE ::
>> I have not used gcc optimizations while compilation.
>> With O2 level optimization proposed solution is taking more time.
>
> The thing is, the kernel is compiled with -O2, so that's what matters.
>
> Also, for very tight loops like this, the major costs tend to be very
> subtle microarchitectural details, particularly branch prediction.
> Which in turn end up sometimes depending on just exactly where the
> branches were placed, and even whether two conditional branches were
> in the same 8-byte aligned region etc things (because the branch
> prediction might be done ignoring the low bits of the EIP etc). So not
> only does the exact microarchitecture matter, things that don't *seem*
> like they should matter can change behavior a lot.
>
> My point is really that the performance numbers are very ambiguous.
> The patch may well help in some situations, but hurt in others.
>
>                       Linus
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