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

[Manfred Spraul <manfred@colorfullife.com>]

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