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

[Christophe Leroy]

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@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:

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