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Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
From:   Jens Axboe <axboe@kernel.dk>
To:     Jeff Moyer <jmoyer@redhat.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Dan Carpenter <dan.carpenter@oracle.com>,
        kent.overstreet@gmail.com
References: <20181128183531.5139-1-willy@infradead.org>
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 <20181211175156.GF6830@bombadil.infradead.org>
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Message-ID: <6b9a45c4-47a2-4c44-aa7e-6e5e90eff9df@kernel.dk>
Date:   Tue, 11 Dec 2018 11:32:54 -0700
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On 12/11/18 11:05 AM, Jens Axboe wrote:
> On 12/11/18 11:02 AM, Jeff Moyer wrote:
>> Matthew Wilcox <willy@infradead.org> 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