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

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Date:   Fri, 16 Aug 2019 16:57:19 +0200
From:   Jan Kara <jack@...e.cz>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     Jan Kara <jack@...e.cz>, Joseph Qi <jiangqi903@...il.com>,
        Dave Chinner <david@...morbit.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 Fri 16-08-19 21:23:24, Joseph Qi wrote:
> On 19/8/15 23:13, Jan Kara wrote:
> > 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?
> > 
> Yes, I've just used default mount options when testing. And it is indeed
> that there is performance improvement with dioread_nolock after reverting
> the 3 related commits.
> I'll do a supplementary test with parallel dio reads as well as
> dioread_nolock and send out the test result.
> 
> > 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.
> Do you mean mark dioread_nolock as default?

Yes, in fact I'd like to just remove the other path so that we can remove
this confusing mount option.

								Honza

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