Re: [PATCH] aio: Convert ioctx_table to XArray

From: Jens Axboe
Date: Tue Dec 11 2018 - 13:33:02 EST

On 12/11/18 11:05 AM, Jens Axboe wrote:
> 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?).
>> 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...

Don't see any regressions. But if we're fiddling with it anyway, can't
we do something smarter? Make the fast path just index a table, and put
all the big hammers in setup/destroy. We're spending a non-substantial
amount of time doing lookups, that's really no different before and
after the patch.

Jens Axboe