linux-kernel - Re: [PATCH] flex_array: Avoid divisions when accessing elements.

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Message-ID: <BANLkTinZkiy-EMdiJhOu1J2RfO8xDU5OXg@mail.gmail.com>
Date:	Thu, 28 Apr 2011 18:48:32 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] flex_array: Avoid divisions when accessing elements.

On Wed, Apr 27, 2011 at 9:13 AM, Dave Hansen <dave@...ux.vnet.ibm.com> wrote:
> On Tue, 2011-04-26 at 13:41 -0700, Jesse Gross wrote:
>> On most architectures division is an expensive operation and
>> accessing an element currently requires four of them.  This
>> performance penalty effectively precludes flex arrays from
>> being used on any kind of fast path.  However, two of these
>> divisions can be handled at creation time and the others can
>> be replaced by a reciprocal divide, completely avoiding real
>> divisions on access.
>
> flex_array.c has a nice table for how many objects can be allocated:
>
>  * Element size | Objects | Objects |
>  * PAGE_SIZE=4k |  32-bit |  64-bit |
>  * ---------------------------------|
>  *      1 bytes | 4186112 | 2093056 |
>  *      2 bytes | 2093056 | 1046528 |
>  *      3 bytes | 1395030 |  697515 |
>  *      4 bytes | 1046528 |  523264 |
>  *     32 bytes |  130816 |   65408 |
>  *     33 bytes |  126728 |   63364 |
>  *   2048 bytes |    2044 |    1022 |
>  *   2049 bytes |    1022 |     511 |
>  *       void * | 1046528 |  261632 |
>
> This patch changes that a bit.  Would you mind updating it?

Sure, I'll send out an updated patch that does that tomorrow.
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