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.le...@csgroup.eu> > >>>> Signed-off-by: Christoph Hellwig <h...@lst.de> > >>> > >>> Tested-by: Christophe Leroy <christophe.le...@csgroup.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
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. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)