linux-kernel - RE: [PATCH] xfs: Fix 64-bit division on 32-bit in xlog_state_switch

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Message-ID: <7478840c18fc45379697609757c6c747@AcuMS.aculab.com>
Date:   Mon, 14 Jun 2021 08:18:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Geert Uytterhoeven' <geert@...ux-m68k.org>,
        Dave Chinner <david@...morbit.com>
CC:     Dave Chinner <dchinner@...hat.com>,
        Chandan Babu R <chandanrlinux@...il.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        "Allison Henderson" <allison.henderson@...cle.com>,
        Christoph Hellwig <hch@....de>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        Linux-Next <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "noreply@...erman.id.au" <noreply@...erman.id.au>
Subject: RE: [PATCH] xfs: Fix 64-bit division on 32-bit in
 xlog_state_switch_iclogs()

From: Geert Uytterhoeven
> Sent: 11 June 2021 07:55
> Hi Dave,
> 
> On Fri, Jun 11, 2021 at 12:02 AM Dave Chinner <david@...morbit.com> wrote:
> > On Thu, Jun 10, 2021 at 01:00:01PM +0200, Geert Uytterhoeven wrote:
...
> > 64 bit division on 32 bit platforms is still a problem in this day
> > and age?
> 
> They're not a problem.  But you should use the right operations from
> <linux/math64.h>, iff you really need these expensive operations.

(64bit) division isn't exactly cheap on 64bit cpus.

Some timing tables for x86 give latencies of well over 1 bit/clock
for Intel cpus, AMD ryzen manage 2 bits/clock.
Signed divide is also significantly more expensive than
unsigned divide.

Integer divide performance is clearly not important enough
to throw silicon at.
The same tables show fdiv having a latency of 16 clocks.

	David

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