linux-kernel - Re: Slow file transfer speeds with CFQ IO scheduler in some cases

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Message-ID: <x49vdudc60z.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 24 Nov 2008 13:50:36 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	"Vitaly V. Bursov" <vitalyb@...enet.dn.ua>,
	linux-kernel@...r.kernel.org
Subject: Re: Slow file transfer speeds with CFQ IO scheduler in some cases

Jens Axboe <jens.axboe@...cle.com> writes:

> On Mon, Nov 24 2008, Jeff Moyer wrote:
>> Jens Axboe <jens.axboe@...cle.com> writes:
>> 
>> > nfsd aside (which does seem to have some different behaviour skewing the
>> > results), the original patch came about because dump(8) has a really
>> > stupid design that offloads IO to a number of processes. This basically
>> > makes fairly sequential IO more random with CFQ, since each process gets
>> > its own io context. My feeling is that we should fix dump instead of
>> > introducing a fair bit of complexity (and slowdown) in CFQ. I'm not
>> > aware of any other good programs out there that would do something
>> > similar, so I don't think there's a lot of merrit to spending cycles on
>> > detecting cooperating processes.
>> >
>> > Jeff will take a look at fixing dump instead, and I may have promised
>> > him that santa will bring him something nice this year if he does (since
>> > I'm sure it'll be painful on the eyes).
>> 
>> Sorry to drum up this topic once again, but we've recently run into
>> another instance where the close cooperator patch helps significantly.
>> The case is KVM using the virtio disk driver.  The host-side uses
>> posix_aio calls to issue I/O on behalf of the guest.  It's worth noting
>> that pthread_create does not pass CLONE_IO (at least that was my reading
>> of the code).  It is questionable whether it really should as that will
>> change the I/O scheduling dynamics.
>> 
>> So, Jens, what do you think?  Should we collect some performance numbers
>> to make sure that the close cooperator patch doesn't hurt the common
>> case?
>
> No, posix aio is a piece of crap on Linux/glibc so we want to be fixing
> that instead. A quick fix is again to use CLONE_IO, though posix aio
> needs more work than that. I told the qemu guys not to use posix aio a
> long time ago since it does stink and doesn't perform well under any
> circumstance... So I don't consider that a valid use case, there's a
> reason that basically nobody is using posix aio.

It doesn't help that we never took in patches to the kernel that would
allow for a usable posix aio implementation, but I digress.

My question to you is how many use cases do we dismiss as broken before
recognizing that people actually do this, and that we should at least
try to detect and gracefully deal with it?  Is this too much to expect
from the default I/O scheduler?  Sorry to beat a dead horse, but folks
do view this as a regression, and they won't be changing their
applications, they'll be switching I/O schedulers to fix this.

Cheers,
Jeff
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