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SubjectRe: CFQ idling kills I/O performance on ext4 with blkio cgroup controller
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> Il giorno 18 mag 2019, alle ore 21:28, Theodore Ts'o <tytso@mit.edu> ha scritto:
>
> 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.
>
> Hi Paolo, I'm curious if you could give a quick summary about what you
> changed in BFQ?
>

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().
>

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...
>
> 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=ordered 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.
>

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
>
> dd if=/dev/zero of=/root/test.img bs=512 count=10000 oflags=dsync
>
> 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.
>
> - Ted

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