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

Rong Chen
2020-01-08T02:31:55Z

On 1/8/20 1:28 AM, Jan Kara wrote:
> 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

We're sorry for the misunderstanding, "-20.2%" means the change of
filebench.sum_bytes_mb/s,
"regression" means the explanation of this change from LKP.

Best Regards,
Rong Chen