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From: zhihua che <zhihua.che@gmail.com>
To: linux-kernel@vger.kernel.org
Subject: Slub Allocator: Why get_order(size * MAX_OBJS_PER_PAGE) - 1 in function slab_order()?
Date: Wed, 23 Nov 2011 04:40:36 +0800	[thread overview]
Message-ID: <CABexPfF-31f1vcJpqdsDHKUHfes1230vwGMtm6ETZXVQQf4bcA@mail.gmail.com> (raw)

Hi, everyone,
        I'm reading the kernel codes about slub allocator and I come
across a confusion. Precisely, I'm reading the initialization of the
slub allocator, kmem_cache_init(), and I find it needs call
calculate_sizes() to determine the order of a kmem_cache, given the
size of the object. In turn, it calls the get_order() to get a
possible order. The problem is, in the start of this function, why it
looks like this:

        if (order_objects(min_order, size, reserved) > MAX_OBJS_PER_PAGE)
                return get_order(size * MAX_OBJS_PER_PAGE) - 1;

        I don't know why it subtracts one from the order returned by
get_order().
        because as far as I know, get_order() returns the order the
slab requires to reserve size * MAX_OBJS_PER_PAGE memory. If it
subtracts 1 from the order returned by get_order(), the slab can't
store MAX_OBJS_PER_PAGE objects at all, instead it can only store half
of the MAX_OBJS_PER_PAGE objects.
        Could you correct me if I think in a wrong way.

             reply	other threads:[~2011-11-22 20:40 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-11-22 20:40 zhihua che [this message]
2011-11-23  2:53 ` Slub Allocator: Why get_order(size * MAX_OBJS_PER_PAGE) - 1 in function slab_order()? David Rientjes
     [not found]   ` <CABexPfEte8U5pVBJ=LvRKMY5=oW7F2vRjw_d9893Yy8W88hfDg@mail.gmail.com>
2011-11-23  6:59     ` Fwd: " zhihua che
2011-11-23 23:06       ` David Rientjes
     [not found]         ` <CABexPfFFiNf8gj9EHnnWABmcK_wCiRkhxQObUnr0_oY2Dkp+JA@mail.gmail.com>
2011-11-24  4:44           ` zhihua che
2011-11-24  4:55             ` zhihua che

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