Re: [PATCH] Radix-tree pagecache for 2.5

[Josh MacDonald <jmacd@CS.Berkeley.EDU>]

Quoting Momchil Velikov (velco@fadata.bg):
> >>>>> "John" == John Stoffel <stoffel@casc.com> writes:
> 
> Momchil> Memory overhead due to allocator overhead is of no concern with the
> Momchil> slab allocator. What matters most is probably the overhead of the
> Momchil> radix tree nodes themselves, compared to the two pointers in struct
> Momchil> page with the hash table approach. rat-4 variant ought to have less
> Momchil> overhead compared to rat-7 at the expense of deeper/higher tree. I
> Momchil> have no figures for the actual memory usage though. For small files it
> Momchil> should be negligible, i.e. one radix tree node, 68 or 516 bytes for
> Momchil> rat-4 or rat-7, for a file of size up to 65536 or 524288 bytes.  The
> Momchil> worst case would be very large file with a few cached pages with
> Momchil> offsets uniformly distributed across the whole file, that is having
> Momchil> deep tree with only one page hanging off each leaf node.
> 
> John> Isn't this a good place to use AVL trees then, since they balance
> John> automatically?  Admittedly, it may be more overhead than we want in
> John> the case where the tree is balanced by default anyway.  
> 
> The widespread opinion is that binary trees are generally way too deep
> compared to radix trees, so searches have larger cache footprint.

I've posted this before -- my cache-optimized skip list solves the
problem of balanced-tree cache footprint.  It uses cacheline-sized
nodes and per-node locking to avoid false-sharing and increase 
concurrency.  The memory usage for the skip list is also less than
the red-black tree for trees larger than several hundred nodes.

I posted a graph on space consumption (using the Linux vm_area_struct to 
calculate space overhead) at:

	http://prdownloads.sourceforge.net/skiplist/slrb_space.gif

There are also results for concurrency and performance as a function 
of node size.

-josh

-- 
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