linux-kernel - Re: [PATCH] SLAB : use a multiply instead of a divide in obj_to

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Message-ID: <20061205144159.GB4388@ucw.cz>
Date:	Tue, 5 Dec 2006 14:42:00 +0000
From:	Pavel Machek <pavel@....cz>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Andrew Morton <akpm@...l.org>, David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, Christoph Lameter <clameter@....com>
Subject: Re: [PATCH] SLAB : use a multiply instead of a divide in obj_to_index()

Hi!

> When some objects are allocated by one CPU but freed by another CPU we can 
> consume lot of cycles doing divides in obj_to_index().
> 
> (Typical load on a dual processor machine where network interrupts are handled 
> by one particular CPU (allocating skbufs), and the other CPU is running the 
> application (consuming and freeing skbufs))
> 
> Here on one production server (dual-core AMD Opteron 285), I noticed this 
> divide took 1.20 % of CPU_CLK_UNHALTED events in kernel. But Opteron are 
> quite modern cpus and the divide is much more expensive on oldest 
> architectures :
> 
> On a 200 MHz sparcv9 machine, the division takes 64 cycles instead of 1 cycle 
> for a multiply.
> 
> Doing some math, we can use a reciprocal multiplication instead of a divide.
> 
> If we want to compute V = (A / B)  (A and B being u32 quantities)
> we can instead use :
> 
> V = ((u64)A * RECIPROCAL(B)) >> 32 ;
> 
> where RECIPROCAL(B) is precalculated to ((1LL << 32) + (B - 1)) / B

Well, I guess it should be gcc doing this optimalization, not we by
hand. And I believe gcc *is* smart enough to do it in some cases...

							pavel

-- 
Thanks for all the (sleeping) penguins.
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