linux-kernel - Re: [PATCHv4 04/39] radix-tree: implement preload for multiple contiguous elements

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Message-Id: <20130522120356.9CB12E0090@blue.fi.intel.com>
Date:	Wed, 22 May 2013 15:03:56 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Dave Hansen <dave@...1.net>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	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: [PATCHv4 04/39] radix-tree: implement preload for multiple
 contiguous elements

Dave Hansen wrote:
> On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> > This patch introduces radix_tree_preload_count(). It allows to
> > preallocate nodes enough to insert a number of *contiguous* elements.
> 
> Would radix_tree_preload_contig() be a better name, then?

Yes. Will rename.

> ...
> > 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.
> 
> Thanks for adding that to the description.  The array you're talking
> about is just pointers, right?
> 
> 107-43 = 64.  So, we have 64 extra pointers * NR_CPUS, plus 64 extra
> radix tree nodes that we will keep around most of the time.  On x86_64,
> that's 512 bytes plus 64*560 bytes of nodes which is ~35k of memory per CPU.
> 
> That's not bad I guess, but I do bet it's something that some folks want
> to configure out.  Please make sure to call out the actual size cost in
> bytes per CPU in future patch postings, at least for the common case
> (64-bit non-CONFIG_BASE_SMALL).

I will add this to the commit message:

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?

I probably, will add !BASE_SMALL dependency to
TRANSPARENT_HUGEPAGE_PAGECACHE config option.

> 
> > Since only THP uses batched preload at the , we disable (set max preload
> > to 1) it if !CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE. This can be changed
> > in the future.
> 
> "at the..."  Is there something missing in that sentence?

at the moment :)

> No major nits, so:
> 
> Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>

Thanks!

-- 
 Kirill A. Shutemov
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