From: Jens Axboe <axboe@kernel.dk>
To: Bart Van Assche <bvanassche@acm.org>,
I Hsin Cheng <richard120310@gmail.com>
Cc: akpm@linux-foundation.org, linux-block@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] blk-wbt: Speed up integer square root in rwb_arm_timer
Date: Fri, 29 Mar 2024 13:12:56 -0600 [thread overview]
Message-ID: <be8e14ec-3d46-4d67-88b0-f3dbf7ff22b2@kernel.dk> (raw)
In-Reply-To: <848d1259-ff6e-4732-b840-a02a5e5fe2cb@acm.org>
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
next prev parent reply other threads:[~2024-03-29 19:12 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-29 9:12 [PATCH] blk-wbt: Speed up integer square root in rwb_arm_timer I Hsin Cheng
2024-03-29 18:15 ` Bart Van Assche
2024-03-29 19:12 ` Jens Axboe [this message]
[not found] ` <CAH5jb=btggiO7Op+k2pTS8LyjcZ0tYpvtWJRmQ8sApD-fHyHXg@mail.gmail.com>
2024-03-30 8:45 ` I Hsin Cheng
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