netdev - RE: [PATCH net-next] net: sched: use no more than one page in struct fw

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Message-ID: <1395069365.9668.49.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 17 Mar 2014 08:16:05 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Thomas Graf <tgraf@...g.ch>, David Miller <davem@...emloft.net>,
	John Fastabend <john.fastabend@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next] net: sched: use no more than one page in
 struct fw_head

On Mon, 2014-03-17 at 14:29 +0000, David Laight wrote:

> Especially for a modulus operation - which requires a second multiply
> and probably has issues with some large values.
> 
> For a hash you could use '(handle * 0x1ffull) >> 32' to reduce the hash.

Nice try, but this wont work.

Most of the time @handle is a small value, for example in the 0 .. 100
range.


> 
> or use a 'modulo 2^n-1 reduction':
> 	handle = (handle & 0x3ffff) + (handle >> 18);
> 	do
> 		handle = (handle & 0x1ff) + (handle >> 9);
> 	while (handle > 0x1ffu);

All these tricks were needed 20 years ago.

Even Thomas laptop can do a divide faster than this loop.

Let the compiler do the thing itself, it is its job.


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