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Date:   Tue, 11 Dec 2018 09:51:56 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Jeff Moyer <jmoyer@redhat.com>
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>
Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
Message-ID: <20181211175156.GF6830@bombadil.infradead.org>
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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.

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?