Re: [ext4] b1b4705d54: filebench.sum_bytes_mb/s -20.2% regression

Jan Kara
2020-01-07T17:28:28Z

On Tue 07-01-20 11:57:08, Theodore Y. Ts'o wrote:
> On Tue, Jan 07, 2020 at 02:41:06PM +0100, Jan Kara wrote:
> > Hello,
> >
> > On Tue 24-12-19 08:59:15, kernel test robot wrote:
> > > FYI, we noticed a -20.2% regression of filebench.sum_bytes_mb/s due to commit:
> > >
> > >
> > > commit: b1b4705d54abedfd69dcdf42779c521aa1e0fbd3 ("ext4: introduce direct I/O read using iomap infrastructure")
> > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > >
> > > in testcase: filebench
> > > on test machine: 8 threads Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz with 8G memory
> > > with following parameters:
> > >
> > > disk: 1HDD
> > > fs: ext4
> > > test: fivestreamreaddirect.f
> > > cpufreq_governor: performance
> > > ucode: 0x27
> >
> > I was trying to reproduce this but I failed with my test VM. I had SATA SSD
> > as a backing store though so maybe that's what makes a difference. Maybe
> > the new code results in somewhat more seeks because the five threads which
> > compete in submitting sequential IO end up being more interleaved?
>
> A "-20.2% regression" should be read as a "20.2% performance
> improvement" is zero-day kernel speak.

Are you sure? I can see:

58.30 ± 2% -20.2% 46.53 filebench.sum_bytes_mb/s

which implies to me previously the throughput was 58 MB/s and after the
commit it was 46 MB/s?

Anyway, in my testing that commit made no difference in that benchmark
whasoever (getting around 97 MB/s for each thread before and after the
commit).

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR