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

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Date:   Sun, 6 Sep 2020 22:09:29 +0200
From:   "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
To:     Pavel Machek <pavel@...x.de>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        David Laight <David.Laight@...lab.com>,
        Christoph Hellwig <hch@....de>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] /dev/zero: also implement ->read

On Sun, Sep 06, 2020 at 08:38:20PM +0200, Pavel Machek wrote:
> 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.

There is nothing wrong with this patch (aside from the sparse warning),
and it's in my tree now, so I don't understand complaining about it...

greg k-h