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Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
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
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <0f77a532-0d88-78bc-b9cc-06bb203a0405@kernel.dk>
Date:   Tue, 11 Dec 2018 11:05:12 -0700
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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...

-- 
Jens Axboe