linux-kernel - Re: [ext4] b1b4705d54: filebench.sum_bytes

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Date:   Tue, 7 Jan 2020 18:28:24 +0100
From:   Jan Kara <jack@...e.cz>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, kernel test robot <rong.a.chen@...el.com>,
        Matthew Bobrowski <mbobrowski@...browski.org>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        lkp@...ts.01.org
Subject: Re: [ext4] b1b4705d54: filebench.sum_bytes_mb/s -20.2% regression

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@...e.com>
SUSE Labs, CR