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List:       linux-kernel
Subject:    Re: Thought about credit_entropy_bits() math
From:       "George Spelvin" <linux () horizon ! com>
Date:       2015-10-21 18:13:16
Message-ID: 20151021181316.25618.qmail () ns ! horizon ! com
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(Resend because I can't spell "kernel.org".)

H. Peter Anvin wrote:
> The main advantage with this approximation is that it doesn't need a
> multiplication instruction.  Instead, it can be implemented with two
> shifts and a subtract on hardware for which multiplication is slow.

Er...  I'm as addicted to micro-optimization as anyone, which is why
I posted all those various approximations, but I'm taking about going
from one to two, not zero to one.

The current approximaion is used right in the middle of a non-constant
multiply:
	unsigned int add = ((pool_size - entropy_count)*anfrac*3) >> s;


(Does Linux even run on any hardware without a multiply instruction?
There are tons of multiplies all over the scheduler.  The worst cases
I can think of just have slow bitwise multiplies: the nommu 68000 and
SPARCv7's multiply step.)
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