linux-kernel - Re: [PATCH] /dev/zero: also implement ->read

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Message-ID: <20200906183820.GA13290@amd>
Date:   Sun, 6 Sep 2020 20:38:20 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Pavel Machek <pavel@...x.de>,
        David Laight <David.Laight@...LAB.COM>,
        Christoph Hellwig <hch@....de>,
        "arnd@...db.de" <arnd@...db.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] /dev/zero: also implement ->read

On Sun 2020-09-06 20:35:38, Christophe Leroy wrote:
> Hi,
> 
> Le 06/09/2020 à 20:21, Pavel Machek a écrit :
> >Hi!
> >
> >>>>Christophe reported a major speedup due to avoiding the iov_iter
> >>>>overhead, so just add this trivial function.  Note that /dev/zero
> >>>>already implements both an iter and non-iter writes so this just
> >>>>makes it more symmetric.
> >>>>
> >>>>Christophe Leroy <christophe.leroy@...roup.eu>
> >>>>Signed-off-by: Christoph Hellwig <hch@....de>
> >>>
> >>>Tested-by: Christophe Leroy <christophe.leroy@...roup.eu>
> >>
> >>Any idea what has happened to make the 'iter' version so bad?
> >
> >Exactly. Also it would be nice to note how the speedup was measured
> >and what the speedup is.
> >
> 
> Was measured on an 8xx powerpc running at 132MHz with:
> 
> 	dd if=/dev/zero of=/dev/null count=1M
> 
> With the patch, dd displays a throughput of 113.5MB/s
> Without the patch it is 99.9MB/s

Actually... that does not seem like a huge deal. read(/dev/zero) is
not that common operation.

Are you getting similar speedups on normal hardware?

								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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