linux-kernel - RE: [PATCH] ext4: convert to DIV_ROUND_UP() in mpage_process_page

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Message-ID: <6135e1e118724e5ea3a1f7640b705cb5@AcuMS.aculab.com>
Date:   Mon, 13 Mar 2023 09:25:20 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Theodore Ts'o' <tytso@....edu>, Yangtao Li <frank.li@...o.com>
CC:     "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>
Subject: RE: [PATCH] ext4: convert to DIV_ROUND_UP() in
 mpage_process_page_bufs()

...
> On 32-bit platforms --- i386 in particular --- the 64-bit division
> results in an out-of-line call to a helper function that is not
> supplied in the kernel compilation environment, so not only is it
> slower, it Just Doesn't Work.

Even on some 64-bit systems a 64bit divide can be significantly
slower than a 32-bit divide - even with the same arguments.
IIRC Intel x86-64 a 64-bit divide (128-bit dividend) is about
twice the clocks of a 32-bit one. On AMD cpu they are much the same.

	David

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