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

>> 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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