linux-kernel - Re: [PATCH] blk-wbt: Speed up integer square root in rwb_arm

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Date: Fri, 29 Mar 2024 13:12:56 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Bart Van Assche <bvanassche@....org>,
 I Hsin Cheng <richard120310@...il.com>
Cc: akpm@...ux-foundation.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-wbt: Speed up integer square root in rwb_arm_timer

On 3/29/24 12:15 PM, Bart Van Assche wrote:
> On 3/29/24 2:12 AM, I Hsin Cheng wrote:
>> As the result shown, the origin version of integer square root, which is
>> "int_sqrt" takes 35.37 msec task-clock, 1,2181,3348 cycles, 1,6095,3665
>> instructions, 2551,2990 branches and causes 1,0616 branch-misses.
>>
>> At the same time, the variant version of integer square root, which is
>> "int_fastsqrt" takes 33.96 msec task-clock, 1,1645,7487 cyclces,
>> 5621,0086 instructions, 321,0409 branches and causes 2407 branch-misses.
>> We can clearly see that "int_fastsqrt" performs faster and better result
>> so it's indeed a faster invariant of integer square root.
> 
> I'm not sure that a 4% performance improvement is sufficient to
> replace the int_sqrt() implementation. Additionally, why to add a
> second implementation of int_sqrt() instead of replacing the
> int_sqrt() implementation in lib/math/int_sqrt.c?

That's the real question imho - if provides the same numbers and is
faster, why have two?

I ran a quick test because I was curious, and the precision is
definitely worse. The claim that it is floor(sqrt(val)) is not true.
Trivial example:

1005117225
	sqrt()		31703.58
	int_sqrt()	30703
	int_fastsqrt()	30821

whether this matters, probably not, but then again it's hard to care
about a slow path sqrt calculation. I'd certainly err on the side of
precision for that.

-- 
Jens Axboe