linux-ext4 - Re: [RFC] performance regression with "ext4: Allow parallel DIO reads"

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Message-ID: <20190815151336.GO14313@quack2.suse.cz>
Date:   Thu, 15 Aug 2019 17:13:36 +0200
From:   Jan Kara <jack@...e.cz>
To:     Joseph Qi <jiangqi903@...il.com>
Cc:     Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Theodore Ts'o <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>,
        Liu Bo <bo.liu@...ux.alibaba.com>
Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO
 reads"

On Tue 30-07-19 09:34:39, Joseph Qi wrote:
> On 19/7/29 06:51, Dave Chinner wrote:
> > On Fri, Jul 26, 2019 at 09:12:07AM +0800, Joseph Qi wrote:
> >>
> >>
> >> On 19/7/26 05:20, Andreas Dilger wrote:
> >>>
> >>>> On Jul 23, 2019, at 5:17 AM, Joseph Qi <jiangqi903@...il.com> wrote:
> >>>>
> >>>> Hi Ted & Jan,
> >>>> Could you please give your valuable comments?
> >>>
> >>> It seems like the original patches should be reverted?  There is no data
> >>
> >> From my test result, yes.
> >> I've also tested libaio with iodepth 16, it behaves the same. Here is the test
> >> data for libaio 4k randrw:
> >>
> >> -------------------------------------------------------------------------------------------
> >> w/ parallel dio reads | READ 78313KB/s, 19578, 1698.70us  | WRITE 78313KB/s, 19578, 4837.60us
> >> -------------------------------------------------------------------------------------------
> >> w/o parallel dio reads| READ 387774KB/s, 96943, 1009.73us | WRITE 387656KB/s，96914, 308.87us
> >> -------------------------------------------------------------------------------------------
> >>
> >> Since this commit went into upstream long time ago，to be precise, Linux
> >> 4.9, I wonder if someone else has also observed this regression, or
> >> anything I missed?
> > 
> > I suspect that the second part of this set of mods that Jan had
> > planned to do (on the write side to use shared locking as well)
> > did not happen and so the DIO writes are serialising the workload.
> > 
> 
> Thanks for the inputs, Dave.
> Hi Jan, Could you please confirm this?
> If so, should we revert this commit at present?

Sorry for getting to you only now. I was on vacation and then catching up
with various stuff. I suppose you are not using dioread_nolock mount
option, are you? Can you check what are your results with that mount
option?

I have hard time remembering what I was thinking those couple years back
but I think the plan was to switch to dioread_nolock always but somehow I
didn't finish that and now I forgot where I got stuck because I don't see
any problem with that currently.

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