linux-kernel - iter and normal ops on /dev/zero & co, was Re: remove the last set_fs() in common code, and remove it for x86 and powerpc

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Date:   Wed, 19 Aug 2020 09:22:13 +0200
From:   Christoph Hellwig <hch@....de>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, linux-arch@...r.kernel.org,
        Kees Cook <keescook@...omium.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: iter and normal ops on /dev/zero & co, was Re: remove the last
 set_fs() in common code, and remove it for x86 and powerpc

On Wed, Aug 19, 2020 at 09:16:59AM +0200, Christophe Leroy wrote:
> I made a test with only the first patch of your series: That's definitely 
> the culprit. With only that patch applies, the duration is 6.64 seconds, 
> that's a 25% degradation.

For the record: the first patch is:

     mem: remove duplicate ops for /dev/zero and /dev/null

So these micro-optimizations matter at least for some popular
benchmarks.  It would be easy to drop, but that means we either:

 - can't support kernel_read/write on these files, which should not
   matter

or
 
 - have to drop the check for both ops being present

Al, what do you think?