linux-kernel - Re: [PATCHv2, RFC 04/30] radix-tree: implement preload for multiple contiguous elements

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Date:	Fri, 22 Mar 2013 07:38:27 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2, RFC 04/30] radix-tree: implement preload for multiple
 contiguous elements

On 03/22/2013 02:47 AM, Kirill A. Shutemov wrote:
> Dave Hansen wrote:
>> On 03/14/2013 10:50 AM, Kirill A. Shutemov wrote:
>>> +#define RADIX_TREE_PRELOAD_NR		512 /* For THP's benefit */
>>
>> This eventually boils down to making the radix_tree_preload array
>> larger.  Do we really want to do this unconditionally if it's only for
>> THP's benefit?
> 
> It will be useful not only for THP. Batching can be useful to solve
> scalability issues.

Still, it seems like something that little machines with no THP support
probably don't want to pay the cost for.  Perhaps you could enable it
for THP||NR_CPUS>$FOO.

>> For those of us too lazy to go compile a kernel and figure this out in
>> practice, how much bigger does this make the nodes[] array?
> 
> We have three possible RADIX_TREE_MAP_SHIFT:
> 
> #ifdef __KERNEL__
> #define RADIX_TREE_MAP_SHIFT	(CONFIG_BASE_SMALL ? 4 : 6)
> #else
> #define RADIX_TREE_MAP_SHIFT	3	/* For more stressful testing */
> #endif
> 
> On 64-bit system:
> For RADIX_TREE_MAP_SHIFT=3, old array size is 43, new is 107.
> For RADIX_TREE_MAP_SHIFT=4, old array size is 31, new is 63.
> For RADIX_TREE_MAP_SHIFT=6, old array size is 21, new is 30.
> 
> On 32-bit system:
> For RADIX_TREE_MAP_SHIFT=3, old array size is 21, new is 84.
> For RADIX_TREE_MAP_SHIFT=4, old array size is 15, new is 46.
> For RADIX_TREE_MAP_SHIFT=6, old array size is 11, new is 19.
> 
> On most machines we will have RADIX_TREE_MAP_SHIFT=6.

Could you stick that in your patch description?  The total cost is
"array size" * sizeof(void*) * NR_CPUS, right?

-- Dave Hansen, Intel OTC Scalability Team
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