linux-kernel - Re: [PATCH] [patch 4a/4] ipc: sem optimise simple operations

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Message-ID: <4A87ED93.5060104@colorfullife.com>
Date:	Sun, 16 Aug 2009 13:29:23 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Nick Piggin <npiggin@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Nadia Derbey <Nadia.Derbey@...l.net>,
	Pierre Peiffer <peifferp@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [patch 4a/4] ipc: sem optimise simple operations

On 08/16/2009 12:31 PM, Nick Piggin wrote:
> On Sat, Aug 15, 2009 at 06:32:14PM +0200, Manfred Spraul wrote:
>    
>> It depends. After disabling inlining, including all helper functions
>> that differ:
>>
>> My proposal: 301 bytes for update_queue.
>>
>> "simple", only negv: 226 bytes
>> "simple, negv+zero: 354 bytes
>> simple+complex: 526 bytes.
>>
>> Thus with only +-1 simple ops, your version uses less icache. If both
>> +-1 and 0 ops are used, your version uses more icache.
>>      
> Don't forget that in that case, your version is badly suboptimal
> due to the algorithmic complexity.
>    
I know, I mentioned it in the change log:
Waking up one "decrement by one" task is O(1) with your code and 
O(1+<number of waiting-for-zero tasks>) with my code.

This is the price paid for saving memory:
Your version keeps three pointers per semaphore (waiting-for-zero, 
oldest decrement, newest decrement)
My version keeps only two pointers (newest decrement, waiting-for-zero). 
The "oldest decrement" is reconstructed on the fly, and that operation 
is O(<number of waiting-for-zero tasks>).

--
     Manfred
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