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

[Christophe Leroy]

Le 06/09/2020 à 22:52, David Laight a écrit :
> From: Christophe Leroy
> > Sent: 06 September 2020 19:36
> > 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
>
> That in itself isn't a problem.
> What was the throughput before any of these patches?
>
> I just remember another thread about the same test running
> a lot slower after one of the related changes.

> While this speeds up read /dev/zero (which is uncommon)
> if this is needed to get near the old performance then
> the changes to the 'iter' code will affect real workloads.

If you are talking about the tests around the set_fs series from 
Christoph, I identified that the degradation was linked to 
CONFIG_STACKPROTECTOR_STRONG being selected by default, which provides 
unreliable results from one patch to another, GCC doing some strange 
things linked to unrelated changes.

With CONFIG_STACKPROTECTOR set to N, I get stable performance and no 
degradation with any of the patches of the set_fs series.

Christophe