Details
-
New Feature
-
Status: Closed
-
Minor
-
Resolution: Won't Fix
-
0.95.2
-
None
-
None
Description
Currently we use a basic version of LRU to handle block caching. LRU is know to be very susceptible to scan thrashing (not scan resistant), which is a common operation in HBase. 2Q is an efficient caching algorithm that emulates the effectivness of LRU/2 (eviction based not on the last access, but rather the access before the last), but is O(1), rather than O(lg(n)) in complexity.
JD has long been talking about investigating 2Q as it may be far better for HBase than LRU and has been shown to be incredibly useful for traditional database caching on production systems.
One would need to implement 2Q (though the pseudocode in the paper is quite explicit) and then test against the existing cache implementation.
The link to the original paper is here: www.vldb.org/conf/1994/P439.PDF
A short overview of 2Q:
2Q uses two queues (hence the name) and a list of pointers to keep track of cached blocks. The first queue is for new, hot items (Ain). If an item is accessed that isn't in Ain, the coldest block is evicted from Ain and the new item replaces it. Anything accessed in Ain is already stored in memory and kept in Ain.
When a block is evicted from Ain, it is moved to Aout as a pointer. If Aout is full, the oldest element is evicted and replaced with the new pointer.
The key to 2Q comes in that when you access something in Aout, it is reloaded into memory and stored in queue B. If B becomes full, then the coldest block is evicted.
This essentially makes Aout a filter for long-term hot items, based on the size of Aout. The original authors found that while you can tune Aout, it generally performs very well at at "50% of the number of pages as would fit into the buffer", but can be tuned as low as 5% at only a slight cost to responsiveness to changes.