database,caching,memory,operating-system,b-tree , How is cache conscious B+tree stored?


How is cache conscious B+tree stored?

Question:

Tag: database,caching,memory,operating-system,b-tree

I am new to database and wish to implement a cache conscious B+tree. A lot of reading suggest storing nodes and leaves as continuous memory. Is this assuming that when B+tree is created, nodes and leaves are stored in the heap, and then copied into disk by read write operation? Does a cache conscious B+tree tell OS to give it a set of continuous physical pages? I think the answer is no b/c applications shouldn't know about how physical pages are allocated, and continuous memory is referring only to primary memory page?


Answer:

The 'cache-consciousness' bit refers to a special discipline of page layout that tries to maximise the utilisation of the first-level data cache of the CPU, often optimised for a specific cache line size (e.g. 64 bytes).

One standard technique (independent of cache line size) is to use offset-value coding in the indirection vector, often combined with 'poor man's normalised keys' (e.g. typically two or four bytes of key material starting with the first byte that is not shared with the predecessor). This reduces the necessity of accessing data outside the indirection vector - i.e. the actual key data that is kept in a heap elsewhere on the page, and quite a few queries (failed seeks) can be done using only the data contained in the beefed-up indirection vector. This maximises cache utilisation and minimises thrashing.

Other schemes form the elements of the indirection vector into a mini-btree with 'page size' equal to the cache line size.

Yet another scheme divides the indirection vector into sub-blocks of one (or very few) cache lines in size, with prefix truncation (called 'front compression' in some papers) used only within these sub blocks but not across different blocks. Binary search among block 'leaders' is used to identify the target block, which is then scanned linearly in the manner that is typical for prefix-truncated key sequences.

A variation of this scheme stores the block leaders in a mini-index and keeps the sequential sub blocks elsewhere, to improve cache utilisation even further. Needless to say, intra-page space management is an absolute nightmare.

Many other variations are possible but publications seem to be limited to academic papers trying to prove academic points, and rare peeks into the page layouts used by important database systems.

Even for something as basic as key comparison in connection with prefix truncation, the only serious reference on the web that I could find dates back to 1990:

DDJ December 1990 - Supercharging Sequential Searches

Papers with good overviews of CPU cache issues with btrees:

Consciousness of the paged nature of the underlying storage - and differing characteristics of seek, page read/write and bulk read/write - is a different beast but also important. It often in results in surprising, innovative designs. One example is Berkeley DB's Java version, which persists only its log file to disk and reconstructs the in-memory btree from the log at startup.


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