linux-kernel - Re: [PATCH] ida: fix up bitmap size calculation

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Message-ID: <AANLkTi=Ema3mTkU7J7XKqmntA+VDJizhBuDHHnZtV_9K@mail.gmail.com>
Date:	Fri, 3 Sep 2010 02:25:22 +0900
From:	Namhyung Kim <namhyung@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ida: fix up bitmap size calculation

Hi,

On Fri, Sep 3, 2010 at 02:20, Tejun Heo <tj@...nel.org> wrote:
> The -1 when calculating IDA_BITMAP_LONGS is intentional.  Please see
> below.
>
> struct ida_bitmap {
>        long                    nr_busy;
>        unsigned long           bitmap[IDA_BITMAP_LONGS];
> };
>
> With the -1, sizeof(struct ida_bitmap) becomes 128 bytes but if you
> remove the -1, it becomes 132 bytes.  Because ida_bitmap doesn't use
> dedicated slab (doesn't make sense as it's not a very hot data
> structure), with 132 bytes, it's gonna be allocated from 256 byte slot
> wasting ~50% of memory.  Adding a comment there to clarify why -1 is
> there would be nice tho.
>

Got it. :-) I'll prepare new patch with that comment, then.
Thank you for the explanation.

-- 
Regards,
Namhyung Kim
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