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

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Date:   Fri, 2 Aug 2019 16:43:04 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        "Berrocal, Eduardo" <eduardo.berrocal@...el.com>
Subject: Re: dax writes on ext4 slower than direct-i/o?

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.

								Honza

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