Try to reduce contention between readers & writer in NodeDB #2
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This makes it possible to persist leaf node values separately from the tree nodes themselves. As leaf nodes are loaded their values are read from the external store, when they're persisted to disk the values are omitted. The external value store needs to be updated separately when a key is set or deleted in the MutableTree.
Previously there was a single mutex that had to be acquired by a reader/writer before they could access either the node cache or the underlying DB. These changes introduce a second mutex to synchronize access to the node cache, while the original mutex is now only used to synchronize write access to the underlying DB. Multiple readers may now access the underlying DB concurrently, this should work with GoLevelDB at least since it supports concurrent readers.
The multi-mutex NodeDB is now called NodeDB2, both the new implementation and the old one implement the new NodeDB interface. MutableTree & ImmutableTree have been modified to only interact with the NodeDB interface, so they can be used with either of the two NodeDB implementations.
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Previously there was a single mutex that had to be acquired by a
reader/writer before they could access either the node cache or the
underlying DB. These changes introduce a second mutex to synchronize
access to the node cache, while the original mutex is now only used to
synchronize write access to the underlying DB.
Multiple readers may now access the underlying DB concurrently, this
should work with GoLevelDB at least since it supports concurrent readers.