Re: [PATCH] aio: Convert ioctx_table to XArray

From: Jens Axboe
Date: Tue Dec 11 2018 - 13:37:50 EST

On 12/11/18 11:09 AM, Jeff Moyer wrote:
> Jens Axboe <axboe@xxxxxxxxx> writes:
>> On 12/11/18 11:02 AM, Jeff Moyer wrote:
>>> Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
>>>> On Tue, Dec 11, 2018 at 12:21:52PM -0500, Jeff Moyer wrote:
>>>>> I'm going to submit this version formally. If you're interested in
>>>>> converting the ioctx_table to xarray, you can do that separately from a
>>>>> security fix. I would include a performance analysis with that patch,
>>>>> though. The idea of using a radix tree for the ioctx table was
>>>>> discarded due to performance reasons--see commit db446a08c23d5 ("aio:
>>>>> convert the ioctx list to table lookup v3"). I suspect using the xarray
>>>>> will perform similarly.
>>>> There's a big difference between Octavian's patch and mine. That patch
>>>> indexed into the radix tree by 'ctx_id' directly, which was pretty
>>>> much guaranteed to exhibit some close-to-worst-case behaviour from the
>>>> radix tree due to IDs being sparsely assigned. My patch uses the ring
>>>> ID which _we_ assigned, and so is nicely behaved, being usually a very
>>>> small integer.
>>> OK, good to know. I obviously didn't look too closely at the two.
>>>> What performance analysis would you find compelling? Octavian's original
>>>> fio script:
>>>>> rw=randrw; size=256k ;directory=/mnt/fio; ioengine=libaio; iodepth=1
>>>>> blocksize=1024; numjobs=512; thread; loops=100
>>>>> on an EXT2 filesystem mounted on top of a ramdisk
>>>> or something else?
>>> I think the most common use case is a small number of ioctx-s, so I'd
>>> like to see that use case not regress (that should be easy, right?).
> Bah, I meant a small number of threads doing submit/getevents.
>>> Kent, what were the tests you were using when doing this work? Jens,
>>> since you're doing performance work in this area now, are there any
>>> particular test cases you care about?
>> I can give it a spin, ioctx lookup is in the fast path, and for "classic"
>> aio we do it twice for each IO...
> Thanks!

You can add my reviewed-by/tested-by. Do you want me to carry this one?
I can rebase on top of the aio.c nospec lookup patch, we should do
those separately.

Jens Axboe