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From:   Paolo Valente <paolo.valente@linaro.org>
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Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup
 controller
Date:   Mon, 20 May 2019 12:38:32 +0200
In-Reply-To: <20190518192847.GB14277@mit.edu>
Cc:     "Srivatsa S. Bhat" <srivatsa@csail.mit.edu>,
        linux-fsdevel@vger.kernel.org,
        linux-block <linux-block@vger.kernel.org>,
        linux-ext4@vger.kernel.org, cgroups@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        jmoyer@redhat.com, amakhalov@vmware.com, anishs@vmware.com,
        srivatsab@vmware.com, Andrea Righi <righi.andrea@gmail.com>
To:     Theodore Ts'o <tytso@mit.edu>
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 <1812E450-14EF-4D5A-8F31-668499E13652@linaro.org>
 <20190518192847.GB14277@mit.edu>
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> Il giorno 18 mag 2019, alle ore 21:28, Theodore Ts'o <tytso@mit.edu> =
ha scritto:
>=20
> On Sat, May 18, 2019 at 08:39:54PM +0200, Paolo Valente wrote:
>> I've addressed these issues in my last batch of improvements for
>> BFQ, which landed in the upcoming 5.2. If you give it a try, and
>> still see the problem, then I'll be glad to reproduce it, and
>> hopefully fix it for you.
>=20
> Hi Paolo, I'm curious if you could give a quick summary about what you
> changed in BFQ?
>=20

Here is the idea: while idling for a process, inject I/O from other
processes, at such an extent that no harm is caused to the process for
which we are idling.  Details in this LWN article:
https://lwn.net/Articles/784267/
in section "Improving extra-service injection".

> I was considering adding support so that if userspace calls fsync(2)
> or fdatasync(2), to attach the process's CSS to the transaction, and
> then charge all of the journal metadata writes the process's CSS.  If
> there are multiple fsync's batched into the transaction, the first
> process which forced the early transaction commit would get charged
> the entire journal write.  OTOH, journal writes are sequential I/O, so
> the amount of disk time for writing the journal is going to be
> relatively small, and especially, the fact that work from other
> cgroups is going to be minimal, especially if hadn't issued an
> fsync().
>=20

Yeah, that's a longstanding and difficult instance of the general
too-short-blanket problem.  Jan has already highlighted one of the
main issues in his reply.  I'll add a design issue (from my point of
view): I'd find a little odd that explicit sync transactions have an
owner to charge, while generic buffered writes have not.

I think Andrea Righi addressed related issues in his recent patch
proposal [1], so I've CCed him too.

[1] https://lkml.org/lkml/2019/3/9/220

> In the case where you have three cgroups all issuing fsync(2) and they
> all landed in the same jbd2 transaction thanks to commit batching, in
> the ideal world we would split up the disk time usage equally across
> those three cgroups.  But it's probably not worth doing that...
>=20
> That being said, we probably do need some BFQ support, since in the
> case where we have multiple processes doing buffered writes w/o fsync,
> we do charnge the data=3Dordered writeback to each block cgroup.  =
Worse,
> the commit can't complete until the all of the data integrity
> writebacks have completed.  And if there are N cgroups with dirty
> inodes, and slice_idle set to 8ms, there is going to be 8*N ms worth
> of idle time tacked onto the commit time.
>=20

Jan already wrote part of what I wanted to reply here, so I'll
continue from his reply.

Thanks,
Paolo

> If we charge the journal I/O to the cgroup, and there's only one
> process doing the
>=20
>   dd if=3D/dev/zero of=3D/root/test.img bs=3D512 count=3D10000 =
oflags=3Ddsync
>=20
> then we don't need to worry about this failure mode, since both the
> journal I/O and the data writeback will be hitting the same cgroup.
> But that's arguably an artificial use case, and much more commonly
> there will be multiple cgroups all trying to at least some file system
> I/O.
>=20
> 						- Ted


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