From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx142.postini.com [74.125.245.142]) by kanga.kvack.org (Postfix) with SMTP id E8EFD6B0032 for ; Wed, 7 Aug 2013 16:00:39 -0400 (EDT) Date: Wed, 7 Aug 2013 22:00:32 +0200 From: Jan Kara Subject: Re: [PATCH 01/23] radix-tree: implement preload for multiple contiguous elements Message-ID: <20130807200032.GE26516@quack.suse.cz> References: <1375582645-29274-1-git-send-email-kirill.shutemov@linux.intel.com> <1375582645-29274-2-git-send-email-kirill.shutemov@linux.intel.com> <20130805111739.GA25691@quack.suse.cz> <20130807163236.0F17DE0090@blue.fi.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130807163236.0F17DE0090@blue.fi.intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Jan Kara , Andrea Arcangeli , Andrew Morton , Al Viro , Hugh Dickins , Wu Fengguang , Mel Gorman , linux-mm@kvack.org, Andi Kleen , Matthew Wilcox , "Kirill A. Shutemov" , Hillf Danton , Dave Hansen , Ning Qu , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org On Wed 07-08-13 19:32:36, Kirill A. Shutemov wrote: > Jan Kara wrote: > > On Sun 04-08-13 05:17:03, Kirill A. Shutemov wrote: > > > From: "Kirill A. Shutemov" > > > > > > The radix tree is variable-height, so an insert operation not only has > > > to build the branch to its corresponding item, it also has to build the > > > branch to existing items if the size has to be increased (by > > > radix_tree_extend). > > > > > > The worst case is a zero height tree with just a single item at index 0, > > > and then inserting an item at index ULONG_MAX. This requires 2 new branches > > > of RADIX_TREE_MAX_PATH size to be created, with only the root node shared. > > > > > > Radix tree is usually protected by spin lock. It means we want to > > > pre-allocate required memory before taking the lock. > > > > > > Currently radix_tree_preload() only guarantees enough nodes to insert > > > one element. It's a hard limit. For transparent huge page cache we want > > > to insert HPAGE_PMD_NR (512 on x86-64) entries to address_space at once. > > > > > > This patch introduces radix_tree_preload_count(). It allows to > > > preallocate nodes enough to insert a number of *contiguous* elements. > > > The feature costs about 5KiB per-CPU, details below. > > > > > > Worst case for adding N contiguous items is adding entries at indexes > > > (ULONG_MAX - N) to ULONG_MAX. It requires nodes to insert single worst-case > > > item plus extra nodes if you cross the boundary from one node to the next. > > > > > > Preload uses per-CPU array to store nodes. The total cost of preload is > > > "array size" * sizeof(void*) * NR_CPUS. We want to increase array size > > > to be able to handle 512 entries at once. > > > > > > Size of array depends on system bitness and on RADIX_TREE_MAP_SHIFT. > > > > > > 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. 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 > > > > > > Since only THP uses batched preload at the moment, we disable (set max > > > preload to 1) it if !CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE. This can be > > > changed in the future. > > > > > > Signed-off-by: Matthew Wilcox > > > Signed-off-by: Kirill A. Shutemov > > > Acked-by: Dave Hansen > > > --- > > > include/linux/radix-tree.h | 11 +++++++++++ > > > lib/radix-tree.c | 41 ++++++++++++++++++++++++++++++++--------- > > > 2 files changed, 43 insertions(+), 9 deletions(-) > > ... > > > diff --git a/lib/radix-tree.c b/lib/radix-tree.c > > > index 7811ed3..99ab73c 100644 > > > --- a/lib/radix-tree.c > > > +++ b/lib/radix-tree.c > > > @@ -82,16 +82,24 @@ static struct kmem_cache *radix_tree_node_cachep; > > > * The worst case is a zero height tree with just a single item at index 0, > > > * and then inserting an item at index ULONG_MAX. This requires 2 new branches > > > * of RADIX_TREE_MAX_PATH size to be created, with only the root node shared. > > > + * > > > + * Worst case for adding N contiguous items is adding entries at indexes > > > + * (ULONG_MAX - N) to ULONG_MAX. It requires nodes to insert single worst-case > > > + * item plus extra nodes if you cross the boundary from one node to the next. > > > + * > > > * Hence: > > > */ > > > -#define RADIX_TREE_PRELOAD_SIZE (RADIX_TREE_MAX_PATH * 2 - 1) > > > +#define RADIX_TREE_PRELOAD_MIN (RADIX_TREE_MAX_PATH * 2 - 1) > > > +#define RADIX_TREE_PRELOAD_MAX \ > > > + (RADIX_TREE_PRELOAD_MIN + \ > > > + DIV_ROUND_UP(RADIX_TREE_PRELOAD_NR - 1, RADIX_TREE_MAP_SIZE)) > > Umm, is this really correct? I see two problems: > > 1) You may need internal tree nodes at various levels but you seem to > > account only for the level 1. > > 2) The rounding doesn't seem right because RADIX_TREE_MAP_SIZE+2 nodes may > > require 3 nodes at level 1 if the indexes are like: > > i_0 | i_1 .. i_{RADIX_TREE_MAP_SIZE} | i_{RADIX_TREE_MAP_SIZE+1} > > ^ ^ > > node boundary node boundary > > My bad. Let's try to calculate once again. > > We want to insert N contiguous items without restriction on alignment. > > Let's limit N <= 1UL << (2 * RADIX_TREE_MAP_SHIFT), without > CONFIG_BASE_SMALL it's 4096. It will simplify calculation a bit. > > Worst case scenario, I can imagine, is tree with only one element at index > 0 and we add N items where at least one index requires max tree high and > we cross boundary between items in root node. > > Basically, at least one index is less then > > 1UL << ((RADIX_TREE_MAX_PATH - 1) * RADIX_TREE_MAP_SHIFT) > > and one equal or more. > > In this case we need: > > - RADIX_TREE_MAX_PATH nodes to build new path to item with index 0; > - DIV_ROUND_UP(N, RADIX_TREE_MAP_SIZE) nodes for last level nodes for new > items; Here, I think you need to count with DIV_ROUND_UP(N + RADIX_TREE_MAP_SIZE - 1, RADIX_TREE_MAP_SIZE) to propely account for the situation b) I described. > - 2 * (RADIX_TREE_MAX_PATH - 2) nodes to build path to last level nodes. > (-2, because root node and last level nodes are already accounted). > > The macro: > > #define RADIX_TREE_PRELOAD_MAX \ > ( RADIX_TREE_MAX_PATH + \ > DIV_ROUND_UP(RADIX_TREE_PRELOAD_NR, RADIX_TREE_MAP_SIZE) + \ > 2 * (RADIX_TREE_MAX_PATH - 2) ) > > For 64-bit system and N=512, RADIX_TREE_PRELOAD_MAX is 37. > > Does it sound correct? Any thoughts? Otherwise I agree with your math (given the limitation on N). But please add a comment explaining how we arrived to the math in RADIX_TREE_PRELOAD_MAX. Thanks! Another option I could live with is the one Matthew described - rely on inserted range being aligned, but then comment about it and add some assertion checking it. Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org