Re: [PATCH] lib : provide a more precise radix_tree_gang_lookup_slot

From: Huang Shijie
Date: Wed May 27 2009 - 03:44:58 EST

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 355f6e8..03e25f4 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -164,7 +164,8 @@ radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
unsigned long first_index, unsigned int max_items);
unsigned int
radix_tree_gang_lookup_slot(struct radix_tree_root *root, void ***results,
- unsigned long first_index, unsigned int max_items);
+ unsigned long first_index, unsigned int max_items,
+ int contig);

Variable `contig' could have the type `bool'. Did you consider and
reject that option, or just didn't think of it?

Yes, type `bool' is better.
+ if (contig)
+ goto out;
+ } else if (contig) {
+ index--;
+ goto out;
+ if (contig) {
+ if (slots_found == 0)
+ break;
+ if (next_index & RADIX_TREE_MAP_MASK)
+ break;
+ }
- (void ***)pages, start, nr_pages);
+ (void ***)pages, start, nr_pages, 0);
- (void ***)pages, index, nr_pages);
+ (void ***)pages, index, nr_pages, 1);

The patch adds cycles in some cases and saves them in others.

Does the saving exceed the adding? How do we know that the patch is a
net benefit?

Assume that:
f0 = called frequency of find_get_pages() (contig == 0)
f1 = called frequency of find_get_pages_contig() (contig == 1)

The primary user of find_get_pages() is ->writepage[s] of some file systems such as ext4.
( I think the shmem_lock() ,truncate() run occasionally which also call it.)

The primary user of find_get_pages_contig() is also the ->writepage[s] of some filesystem
such as afs.( I am not sure whether btrfs is also the main user of it )

So if (f0 nearly equal f1)
cycles saving >> cycles adding

__lookup() saves much cycles when there are holes and the contig==1.

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