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

[Christophe Leroy]

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@xxxxxxxxxx>
> > > > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > >
> > > Tested-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> >
> > 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