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

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Message-ID: <Pine.LNX.4.64.0703060853050.5963@woody.linux-foundation.org>
Date:	Tue, 6 Mar 2007 09:09:18 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Eric Dumazet <dada1@...mosbay.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 Tue, 6 Mar 2007, H. Peter Anvin wrote:
> 
> That's true for *any* sprintf(), though.  sprintf() converts all its arguments
> to 64 bits.

Well, it very much uses "do_div()", so that it can do a

	64 / 32 -> (div64,mod32) 

divide, which is often faster than a full 64:64 divide.

> However, this could be optimized.  I think right now sprintf() uses a generic
> divide-by-base, but a divide by 8 and 16 can of course be handled with a
> shift, and divide by 10 can be replaced with a multiplication by
> 0x1999999999999999ULL on most architectures.

Nope. You need both the result of the division *and* the remainder, and 
you can't do that with a single multiply.

Also, with modern hardware, divides are usually fairly cheap, with early 
exit logic, so that the common case of small integers is fairly cheap. 
Yeah, generating a full 64-bit number printout is still expensive, of 
course (both because you need to do many divides *and* because only the 
last few divides will be able to do any appreciable early exit logic.

Anyway, I think a full integer divide on Core 2 is something like 22 
cycles. Yes, the multiply is much fasster (at 4 cycles), but I think that 
22 cycles is actually worst-case.

Somebody who has a benchmark could try.

		Linus
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