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

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Message-ID: <8c353864-76a9-90bf-fa2f-f7a8231b5487@csgroup.eu>
Date:   Sun, 6 Sep 2020 20:35:38 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Pavel Machek <pavel@...x.de>,
        David Laight <David.Laight@...LAB.COM>
Cc:     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

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

Christophe