From: zhihua che <zhihua.che@gmail.com>
To: linux-kernel@vger.kernel.org
Subject: Re: Slub Allocator: Why get_order(size * MAX_OBJS_PER_PAGE) - 1 in function slab_order()?
Date: Thu, 24 Nov 2011 12:55:55 +0800 [thread overview]
Message-ID: <CABexPfGSdUAfwFC2HBtVTZe=EbSj8Q2Pt_AiaoAOB+ms7rftwQ@mail.gmail.com> (raw)
In-Reply-To: <CABexPfEa-kTdH3=7pa=geVE5rUSiuj2Gzt6Zbjz_-cwXJVYGFA@mail.gmail.com>
2011/11/24 zhihua che <zhihua.che@gmail.com>:
> 2011/11/24 David Rientjes <rientjes@google.com>:
>> On Wed, 23 Nov 2011, zhihua che wrote:
>>
>>> I know what you mean, that is, a slab can only store no more than
>>> MAX_OBJS_PER_PAGE, actually 0x7FFF, objects.
>>>
>>> But get_order(size * MAX_OBJS_PER_PAGE) already returns the order
>>> which reserves no_more_than size * MAX_OBJS_PER_PAGE memory. Right?
>>>
>>
>> Yes, but it reserves too much memory if the conditional is true.
>>
>>> So I think there is no need to subtract one.
>>>
>>
>> If we didn't subtract one, then the order of a slab page would allow for
>> _more_ than MAX_OBJS_PER_PAGE to be allocated and that's not allowed
>> because of the restrictions in struct page.
>>
>> Consider a page size of 4K and an object size of 8 bytes.
>> get_order(8 * 32767) would be 6, so that's a 4K * 2^6 = 256K slab page
>> without the subtraction and could allocate (256K * 1024 / 8) = 32768 which
>> is greater than MAX_OBJS_PER_PAGE and not allowed.
>>
>> So we subtract one so the compound slab page is guaranteed to allocate
>> less than MAX_OBJS_PER_PAGE.
>
> Oh, sorry to bother you ... I guess I calculated a wrong result. I
> repeat your and my example carefully and you're right.
>
> Thanks very much for your patience :)
>
I guess I find what I'm wrong with. I pick a 0x8000 instead of 0x7FFF
as my example, because I though it's convenient.
But the 0x8000, as a power of two, turned out to be a proper order 7,
which can store 0x8000 objects without subtracting one.
Whatever, I made a stupid mistake.
prev parent reply other threads:[~2011-11-24 4:55 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-11-22 20:40 Slub Allocator: Why get_order(size * MAX_OBJS_PER_PAGE) - 1 in function slab_order()? zhihua che
2011-11-23 2:53 ` 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 [this message]
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