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

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Message-ID: <3cae5587-0843-83a9-bf4a-9c1426d499e4@csgroup.eu>
Date:   Sun, 6 Sep 2020 20:47:10 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Pavel Machek <pavel@...x.de>
Cc:     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



Le 06/09/2020 à 20:38, Pavel Machek a écrit :
> 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:

Oops. That was not on an 8xx but on an 8321 running at 333MHz, sorry.

>>
>> 	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.

That's 14% more. It is not negligeable.

I think I need to measure the /dev/zero read standalone. I guess the 
write to /dev/null flatters the result.

> 
> Are you getting similar speedups on normal hardware?
> 

Do you regard powerpc embedded devices as abnormal ?
AFAIK the 832x is embedded in millions of ADSL boxes.
What processor do you have in mind as normal hardware ?

Christophe