Ben Manes commented on HBASE-15560:

Thanks for the reviews so far. When I originally wrote the patch I did enough 
for evaluation and tests seemed premature. I'll see what can be ported over 
from TestLruBlockCache, though I'd expect it to be minimal. I don't think it 
would be appropriate to write tests in HBase that make too many assumptions 
about the policy's behavior as that leads to relying on implementation details 
of an external library.

BlockPriority is coupled to LruBlockCache and arguably it is a leaky 
abstraction by exposing it. I'd argue that the `BlockCache` should be 
redesigned to avoid [dogpiling|https://en.wikipedia.org/wiki/Cache_stampede] by 
computing through the cache and minimize implementation assumptions. 
`MemcachedBlockCache` silently ignores the flag, throws exceptions, and returns 
dummy values for methods that make no sense for it.

If this is merged in, I'd like to see a similar ticket like 
[ACCUMULO-4466|https://issues.apache.org/jira/browse] to evaluate whether to 
make TinyLFU the default. The Lru implementation could then be removed.

> TinyLFU-based BlockCache
> ------------------------
>                 Key: HBASE-15560
>                 URL: https://issues.apache.org/jira/browse/HBASE-15560
>             Project: HBase
>          Issue Type: Improvement
>          Components: BlockCache
>    Affects Versions: 2.0.0
>            Reporter: Ben Manes
>            Assignee: Ben Manes
>         Attachments: HBASE-15560.patch, tinylfu.patch
> LruBlockCache uses the Segmented LRU (SLRU) policy to capture frequency and 
> recency of the working set. It achieves concurrency by using an O( n ) 
> background thread to prioritize the entries and evict. Accessing an entry is 
> O(1) by a hash table lookup, recording its logical access time, and setting a 
> frequency flag. A write is performed in O(1) time by updating the hash table 
> and triggering an async eviction thread. This provides ideal concurrency and 
> minimizes the latencies by penalizing the thread instead of the caller. 
> However the policy does not age the frequencies and may not be resilient to 
> various workload patterns.
> W-TinyLFU ([research paper|http://arxiv.org/pdf/1512.00727.pdf]) records the 
> frequency in a counting sketch, ages periodically by halving the counters, 
> and orders entries by SLRU. An entry is discarded by comparing the frequency 
> of the new arrival (candidate) to the SLRU's victim, and keeping the one with 
> the highest frequency. This allows the operations to be performed in O(1) 
> time and, though the use of a compact sketch, a much larger history is 
> retained beyond the current working set. In a variety of real world traces 
> the policy had [near optimal hit 
> rates|https://github.com/ben-manes/caffeine/wiki/Efficiency].
> Concurrency is achieved by buffering and replaying the operations, similar to 
> a write-ahead log. A read is recorded into a striped ring buffer and writes 
> to a queue. The operations are applied in batches under a try-lock by an 
> asynchronous thread, thereby track the usage pattern without incurring high 
> latencies 
> ([benchmarks|https://github.com/ben-manes/caffeine/wiki/Benchmarks#server-class]).
> In YCSB benchmarks the results were inconclusive. For a large cache (99% hit 
> rates) the two caches have near identical throughput and latencies with 
> LruBlockCache narrowly winning. At medium and small caches, TinyLFU had a 
> 1-4% hit rate improvement and therefore lower latencies. The lack luster 
> result is because a synthetic Zipfian distribution is used, which SLRU 
> performs optimally. In a more varied, real-world workload we'd expect to see 
> improvements by being able to make smarter predictions.
> The provided patch implements BlockCache using the 
> [Caffeine|https://github.com/ben-manes/caffeine] caching library (see 
> HighScalability 
> [article|http://highscalability.com/blog/2016/1/25/design-of-a-modern-cache.html]).
> Edward Bortnikov and Eshcar Hillel have graciously provided guidance for 
> evaluating this patch ([github 
> branch|https://github.com/ben-manes/hbase/tree/tinylfu]).

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