linux-kernel - Re: [GIT PULL] ext4 bug fixes for 3.18

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Message-ID: <CA+55aFz=Dv=FuVD=WDMjOAi-eG2ZbHT5F7Gc78BpMrVDZKpQnQ@mail.gmail.com>
Date:	Sat, 1 Nov 2014 09:29:14 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Theodore Ts'o" <tytso@....edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [GIT PULL] ext4 bug fixes for 3.18

On Sat, Nov 1, 2014 at 6:38 AM, Theodore Ts'o <tytso@....edu> wrote:
>
> I didn't think hash_64 was *that* slow, so it's not clear the above
> would be faster, though.  And if someone is using a > 16TB file system
> on a 32-bit platform, I suspect they might be having other problems.  :-)

Fair enough, hash_64() isn't *that* slow. But it _is_ 6 64-bit shifts
and adds/subtracts, which on a 32-bit machine tends to be quite
expensive. On some of them it's function calls etc.

And your point about >16TB filesystems is completely buggy. That was
*my* point. Most people - even on 64-bit - do *not* have 16TB
filesystems, and the high 32 bits are zero or contain very very little
information (ie even on a multi-terabyte filesystem, it's one or two
bits worth of information). So hash_32() is not only much more
reasonable on a 32-bit machine, the end result is basically as good
for 99.999% of all uses. Exactly *because* people don't have those big
filesystems.

               Linus
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