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Date: Wed, 22 May 2013 07:20:28 -0700
From: Dave Hansen <dave@sr71.net>
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Subject: Re: [PATCHv4 04/39] radix-tree: implement preload for multiple contiguous
 elements
References: <1368321816-17719-1-git-send-email-kirill.shutemov@linux.intel.com> <1368321816-17719-5-git-send-email-kirill.shutemov@linux.intel.com> <519BC3BE.3070702@sr71.net> <20130522120356.9CB12E0090@blue.fi.intel.com>
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Al Viro <viro@zeniv.linux.org.uk>, Hugh Dickins <hughd@google.com>, Wu Fengguang <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>, Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org, Andi Kleen <ak@linux.intel.com>, Matthew Wilcox <matthew.r.wilcox@intel.com>, "Kirill A. Shutemov" <kirill@shutemov.name>, Hillf Danton <dhillf@gmail.com>, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org

On 05/22/2013 05:03 AM, Kirill A. Shutemov wrote:
> On most machines we will have RADIX_TREE_MAP_SHIFT=6. In this case,
> on 64-bit system the per-CPU feature overhead is
>  for preload array:
>    (30 - 21) * sizeof(void*) = 72 bytes
>  plus, if the preload array is full
>    (30 - 21) * sizeof(struct radix_tree_node) = 9 * 560 = 5040 bytes
>  total: 5112 bytes
> 
> on 32-bit system the per-CPU feature overhead is
>  for preload array:
>    (19 - 11) * sizeof(void*) = 32 bytes
>  plus, if the preload array is full
>    (19 - 11) * sizeof(struct radix_tree_node) = 8 * 296 = 2368 bytes
>  total: 2400 bytes
> ---
> 
> Is it good enough?

Yup, just stick the calculations way down in the commit message.  You
can put the description that it "eats about 5k more memory per-cpu than
existing code" up in the very beginning.

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