Re: [PATCH 04/16] radix-tree: look-aside cache

[Nick Piggin]

Wu Fengguang wrote:

> Hi Nick,
>
> On Thu, Nov 10, 2005 at 10:31:09AM +1100, Nick Piggin wrote:
>
> > Does this cache add much performance compared with simple repeated
> > lookups? If the access patterns are highly local, the top of the
> > radix tree should be in cache.
> It just guarantees constant lookup time for small/large files.
>
> My context based read-ahead code has been quite tricky just to avoid many radix
> tree lookups. I made it much simple and robust in the recent versions by just
> scanning through the cache. With the help of look-aside cache, the performance
> remains comparable with the tricky one. Sorry, the oprofile log was overwrote.
> But if you do need some numbers about the cache, I'll make one.

But it isn't *really* constant time lookups? I mean you'll
always have the O(logn) lookup. Amortised I guess that
becomes insignificant?

Briefly: is there a reason why you couldn't use gang lookups
instead? (Sorry I haven't been able to read and understand your
actual readahead code).

> > Do you think you could provide a simple 'use case' for an overview
> > of how you use the cache and what calls to make?
> Ok, here it is:
>
> void func() {
> +       struct radix_tree_cache cache;
> +
> +       radix_tree_cache_init(&cache);
>        read_lock_irq(&mapping->tree_lock);
>        for(;;) {
> -               page = radix_tree_lookup(&mapping->page_tree, index);
> +               page = radix_tree_cache_lookup(&mapping->page_tree, &cache, index);
>        }
>        read_unlock_irq(&mapping->tree_lock);
> }

OK, I guess that seems reasonable.
You have introduced some other APIs as well...

Profile numbers would be great for the cached / non-cached cases.


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