linux-kernel - Re: [PATCH v3 next/akpm] aio: convert the ioctx list to radix tree

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Date:	Tue, 18 Jun 2013 22:05:22 +0300
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	Zach Brown <zab@...hat.com>
Cc:	Benjamin LaHaise <bcrl@...ck.org>,
	Kent Overstreet <koverstreet@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	linux-s390@...r.kernel.org, schwidefsky@...ibm.com,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v3 next/akpm] aio: convert the ioctx list to radix tree

On Fri, Jun 14, 2013 at 5:20 PM, Octavian Purdila
<octavian.purdila@...el.com> wrote:
> On Wed, Jun 12, 2013 at 10:40 PM, Zach Brown <zab@...hat.com> wrote:
>>> I've got an alternate approach for fixing this wart in lookup_ioctx()...
>>> Instead of using an rbtree, just use the reserved id in the ring buffer
>>> header to index an array pointing the ioctx.  It's not finished yet, and
>>> it needs to be tidied up, but is most of the way there.
>>
>> Yeah, that might work.
>>
>> Note that the patch wasn't using an rbtree, it was storing the pointer
>> value in a *radix* which is why single lookups took so long.  Presumably
>> radix was used for RCU lookups.
>>
>> Your ring->id trick lets us use RCU with small ints instead of the
>> context pointer.  It might be worth using idr instead of rolling manual
>> array code.  It'd still be much faster than the list, but get rid of the
>> large alloc, array walking, memcpy(), etc.
>>
>
> I picked up Ben's patch and incorporated Zach's idea and the first
> results look promising, as expected. I am going to do a full test with
> the same workload I've used for rbtree and come back with the results
> and the patch in a day or so.
>

Unfortunately, I still see performance degradation for the one ioctx
case even when using IDR. I am using the same fio benchmark as before.
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