linux-kernel - Re: [PATCH] Fixed a mismatch between the users of radix

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Date:	Mon, 16 Aug 2010 21:35:09 -0700
From:	Salman Qazi <sqazi@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	paulmck@...ibm.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, adurbin@...gle.com
Subject: Re: [PATCH] Fixed a mismatch between the users of radix_tree and the implementation.

On Mon, Aug 16, 2010 at 2:06 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> (html damaged email alert)
>
> On Mon, 2010-08-16 at 13:59 -0700, Salman Qazi wrote:
>> On Mon, Aug 16, 2010 at 12:33 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>         On Mon, 2010-08-16 at 11:30 -0700, Salman Qazi wrote:
>>         > For the delete case,
>>         > we no longer shrink the tree back to being just the root containing the
>>         > only remaining object.  For the insert case, we no longer store the
>>         > first object in the root, rather allocating a node structure for it.  The
>>         > reason that this works is that deleting (or inserting) intermediate nodes
>>         > does not make a difference to a reader holding a slot.
>>
>>
>>         Ah, I through that was what it did. So you basically increase the memory
>>         footprint for tiny files.. have you done any measurements on that?
>>
>
>> You raise a valid concern.  I haven't.  What would you recommend as a
>> benchmark/metric to measure this?
>
> One thing you could try is something like the below on a freshly booted
> machine, once without and once with the patch:
>
>  cd /usr/src/linux-2.6
>  echo 1 > /proc/sys/vm/drop_caches
>  grep radix /proc/slabinfo
>  make bzImage
>  echo 1 > /proc/sys/vm/drop_caches
>  grep radix /proc/slabinfo
>
>
>
>

Here's what I see:

Without the patch:

Before:
radix_tree_node      468   1400    568   28    4 : tunables    0    0
  0 : slabdata     50     50      0

After:
radix_tree_node     1886   3192    568   28    4 : tunables    0    0
  0 : slabdata    114    114      0

With the patch:

Before:

radix_tree_node      495   1176    568   28    4 : tunables    0    0
  0 : slabdata     42     42      0

After:

radix_tree_node     3173   7336    568   28    4 : tunables    0    0
  0 : slabdata    262    262      0


So, not particularly good news :(.
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