[MERGE] Use a dict for Btree internal nodes instead of LRU

[John Arbash Meinel]

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Robert Collins wrote:
> On Fri, 2009-03-27 at 03:28 +0000, Matt Nordhoff wrote:
> 
> 
>> If it's really unlikely that the cache will be filled, but you don't
>> want the performance cost of an LRUCache, why not compromise and use a
>> FIFOCache?
> 
> Sure.
> 
> -Rob

I have a small concern that once your working set is > 1M using a FIFO
cache is going to basically be equivalent to not having a cache.
However, I'll worry about that when we start hitting 1M node trees. We
always cache the root node (in a separate object), so this is only for
the extra internal nodes. If we grow beyond 100, then we'll get another
internal layer anyway. If we just set it to 1000 like the leaf-node
cache, then we would scale into the 10M nodes before it would start
thinking about flushing.

John
=:->

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