Re: [PATCH 3/4] ida: in-place ida allocation

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 04/23/2014 04:02 AM, Tejun Heo wrote:
> On Tue, Apr 22, 2014 at 06:16:20PM +0800, Lai Jiangshan wrote:
>> There are two stages of ida allocation/free, idr_layers and ida_bitmap.
>> They add unneeded foot print and memory waste.
>>
>> When a single ID is first allocated from an ida, this ida requires
>> two big chunks of memory. One idr_layer and one ida_bitmap.
>>
>> To reduce the foot print and memory, we reduce the ida_bitmap
>> to a single "unsigned long" and place it in its own idr-slot
>> and avoid to allocate the ida_bitmap.
>>
>> It also means ida bitmap is located on its coresponding idr-slot
>> which size is the same as "unsigned long".
>> Each ida bitmap(idr-slot) contains BITS_PER_LONG ida-slots.
>>
>> The struct ida_bitmap is not needed any more, we use "unsigned long"
>> directly and remove all the code of alloc/free struct ida_bitmap.
> 
> Are you calling 128 byte a "big chunk of memory" while trading off
> tree depth for it?  No, this level of space optimizaiton is completely
> uncalled for.

I reduce the depth, you forgot that struct ida_bitmap is an additional
depth which is removed in the patch.

> 
>  Nacked-by: Tejun Heo <tj@kernel.org>
> 
> Thanks.
>