linux-kernel - Re: [patch v2] epoll use a single inode ...

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Date:	Tue, 6 Mar 2007 18:28:00 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch v2] epoll use a single inode ...

On Tuesday 06 March 2007 18:19, Linus Torvalds wrote:
> On Tue, 6 Mar 2007, Eric Dumazet wrote:
> > Something like :
> >
> > [PATCH] : Use reciprocal divides in sprintf()
>
> Try this on Core 2, and I suspect that you'll find that the hardware is
> actually *faster* than doing the shift/test, function call and the
> two multiplies.
>

Where do you see a function call ?

     448:       44 89 d0                mov    %r10d,%eax
     44b:       44 89 ea                mov    %r13d,%edx
     44e:       48 0f af c1             imul   %rcx,%rax
     452:       48 c1 e8 20             shr    $0x20,%rax
     456:       0f af d0                imul   %eax,%edx
     459:       49 29 d2                sub    %rdx,%r10
     45c:       43 0f b6 14 16          movzbl (%r14,%r10,1),%edx
     461:       41 89 c2                mov    %eax,%r10d
     464:       41 88 13                mov    %dl,(%r11)
     467:       49 ff c3                inc    %r11
     46a:       85 c0                   test   %eax,%eax
     46c:       75 da                   jne    448 <number+0x138>



> > Using reciprocal divides permits to change each divide by two multiplies,
> > less expensive on current CPUS.
>
> Are you sure?

I am going to test this, but at least on Opterons, the reciprocal divide I 
added into mm/slab.c gave me a nice speedup.

I am going to bench some stupid loop :

for (i = 0 ; i < 1000*1000 ; i++) {
	pipe(fds); close(fds[0]); close(fds[1]);
}

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