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

Christophe Leroy
2020-09-06T18:35:56Z

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

Christophe