2010/4/3, tytso@xxxxxxx <tytso@xxxxxxx>:
> On Wed, Mar 31, 2010 at 10:26:45PM +0800, jing zhang wrote:
>>
>> With the added cache, there is over 50% probability that the operation,
>> rb_first(&(grp->bb_free_root));
>> can be saved, when there are multiple nodes in tree.
>>
>> It seems what is added is following what is called O(1), one of the
>> works by Mr. Ingo Molnar, but I am not sure, and let's ask Mr. Ingo
>> Molnar.
>
> Sure, but does it matter? The red-black tree is per-block group, and
> rb_first() is O(ln n), and it's cleared after every transaction
> commit. Have you measured how deep it gets? Have you measured how
> much CPU time this would actually save?
>
Thanks for these questions, which were out of my consideration.
> I'm almost certiain the code complexity isn't worth it. For example,
> your patch is buggy. There are places where the red black tree is
> manipulated, and where the node pointed at by bb_free_cache could get
> freed. For example, see release_blocks_on_commit() and
> ext4_mb_free_metadata().
I will check release_blocks_on_commit() and ext4_mb_free_metadata()
again next week.
>
> That being said, I'm not convinced ext4_mb_generate_from_freelist() is
> (a) necessary, or (b) bug-free, either. The whole point of having
> extents in bb_free_root tree is that those extents aren't safe to be
> placed in the buddy bitmap. And ext4_mb_generate_from_freelist()
> isn't freeing the nodes from the rbtree. Fortunately it looks like
> ext4_mb_generate_from_freelist is only getting called when the buddy
> bitmap is being set up, so the rbtree should be empty during those
> times.
>
> I need to do some more investigation, but I think the function can be
> removed entirely.
Do you mean that ext4_mb_generate_from_freelist() can be removed entirely?
- zj
>
> - Ted
>
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