linux-ext4 - Re: dax writes on ext4 slower than direct-i/o?

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Message-ID: <CAPcyv4g_q_mzesgXexsSxhE_Shwf-v-uvQOwnCLxw9oBrsdzwA@mail.gmail.com>
Date:   Fri, 2 Aug 2019 08:38:01 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>,
        "Berrocal, Eduardo" <eduardo.berrocal@...el.com>
Subject: Re: dax writes on ext4 slower than direct-i/o?

On Fri, Aug 2, 2019 at 7:43 AM Jan Kara <jack@...e.cz> wrote:
>
> Hi Dan!
>
> On Tue 30-07-19 16:49:41, Dan Williams wrote:
> > Eduardo raised a puzzling question about why dax yields lower iops
> > than direct-i/o. The expectation is the reverse, i.e. that direct-i/o
> > should be slightly slower than dax due to block layer overhead. This
> > holds true for xfs, but on ext4 dax yields half the iops of direct-i/o
> > for an fio 4K random write workload.
> >
> > Here is a relative graph of ext4: dax + direct-i/o vs xfs: dax + direct-i/o
> >
> > https://user-images.githubusercontent.com/56363/62172754-40c01e00-b2e8-11e9-8e4e-29e09940a171.jpg
> >
> > A relative perf profile seems to show more time in
> > ext4_journal_start() which I thought may be due to atime or mtime
> > updates, but those do not seem to be the source of the extra journal
> > I/O.
> >
> > The urgency is a curiosity at this point, but I expect an end user
> > might soon ask whether this is an expected implementation side-effect
> > of dax.
> >
> > Thanks in advance for any insight, and/or experiment ideas for us to go try.
>
> Yeah, I think the reason is that ext4_iomap_begin() currently starts a
> transaction unconditionally for each write whereas ext4_direct_IO_write()
> is more clever and starts a transaction only when needing to allocate any
> blocks. We could put similar smarts into ext4_iomap_begin() and it's
> probably a good idea, just at this moment I'm working with one guy on
> moving ext4 direct IO code to iomap infrastructure which overhauls
> ext4_iomap_begin() anyway, so let's do this after that work.

Sounds good, thanks for the insight!