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

Pavel Machek
2020-09-06T18:38:25Z

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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@csgroup.eu>
> >>>>Signed-off-by: Christoph Hellwig <hch@lst.de>
> >>>
> >>>Tested-by: Christophe Leroy <christophe.leroy@csgroup.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
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