From: Jan Kara <jack@suse.cz>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-ext4 <linux-ext4@vger.kernel.org>, Jan Kara <jack@suse.cz>,
"Berrocal, Eduardo" <eduardo.berrocal@intel.com>
Subject: Re: dax writes on ext4 slower than direct-i/o?
Date: Fri, 2 Aug 2019 16:43:04 +0200 [thread overview]
Message-ID: <20190802144304.GP25064@quack2.suse.cz> (raw)
In-Reply-To: <CAPcyv4g1g2i-9p1ZDqy596O-cbw3Gas2wdiv49EvM+b0i-1uLg@mail.gmail.com>
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@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2019-08-02 14:43 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-30 23:49 dax writes on ext4 slower than direct-i/o? Dan Williams
2019-08-02 14:43 ` Jan Kara [this message]
2019-08-02 15:38 ` Dan Williams
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190802144304.GP25064@quack2.suse.cz \
--to=jack@suse.cz \
--cc=dan.j.williams@intel.com \
--cc=eduardo.berrocal@intel.com \
--cc=linux-ext4@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).