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

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Date:	Tue, 22 Apr 2014 16:02:21 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Jeff Layton <jlayton@...hat.com>,
	Andreas Gruenbacher <agruen@...bit.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Jean Delvare <jdelvare@...e.de>,
	Monam Agarwal <monamagarwal123@...il.com>
Subject: Re: [PATCH 3/4] ida: in-place ida allocation

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.

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

Thanks.

-- 
tejun
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