use SnapTreeMap instead of ConcurrentSkipListMap

[dyf6372]

Using jprofiler, I found that the function in ConcurrentSkipListMap was cost more than 30% cpu in our case.

SnapTreeMap is a drop-in replacement for ConcurrentSkipListMap. Form the paper http://ppl.stanford.edu/papers/ppopp207-bronson.pdf：
Experimental evidence shows that our algorithm outperforms a highly tuned concurrent skip list for many access patterns, with an average of 39% higher singlethreaded throughput and 32% higher multi-threaded throughput over a range of contention levels and operation mixes.

After I changing to SnapTreeMap , it's 20%~30% faster than ConcurrentSkipListMap.

[QiuMM]

Maybe other places that use ConcurrentSkipListMap can be replaced too.

[leventov]

@QiuMM interesting, thanks. FYI, this PR addresses the same problem: #6327

The project's README (https://github.com/nbronson/snaptree) states that the latest released version is 0.2.

[b-slim]

While 35% is spent in the .get call is true but most of that time 34% is consumed by the Druid comparator code, so not sure how the snapTreeMap is going to solve this? can you present some results or benchmarks ?
Plus the last commit in this project is 6 years old and seems that some issues are not fixed https://github.com/nbronson/snaptree/issues @dyf6372 are you aware of other projects using this ?

[dyf6372]

@leventov just like nbronson/snaptree#6 said, v0.2 is not deployed at maven central repo.
@b-slim Cassandra use SnapTreeMap before version 2.1 (https://github.com/apache/cassandra/blob/cassandra-2.0/build.xml). In version 2.1, it used a custom in-memory BTree to replace SnapTreeMap(https://www.datastax.com/dev/blog/cassandra-2-1-now-over-50-faster).

[b-slim]

as per readme

VERSION 0.2: Incorporates an important bugfix to isEmpty() 

I do not think it is a good idea to use code that is missing important bugfixes for such critical
section in Druid.
And as you pointed out Cassandra is not using it anymore.

@dyf6372 seems to me the project is not alive and does not have maintainers thus no one to fix bugs.

[dyf6372]

While 35% is spent in the .get call is true but most of that time 34% is consumed by the Druid comparator code, so not sure how the snapTreeMap is going to solve this? can you present some results or benchmarks ?

@b-slim I'm not sure, I think it may decrease the number of compare or optimize the concurrency. Since the project seems to be not alive, I agree that it's may not a good idea to use this code now. However, it's really necessary for me to improve the performance in index since we have more than 1200 peon task for one cluster and data growth is fast. I'll try it in my cluster and if it's really work without any mistake I'll give the detail result or benchmark in the future.

[b-slim]

@dyf6372 Thanks for doing this.
BTW there is this ongoing effort #6327
Maybe worth taking a look and perhaps joint effort if it make sense.
Happy coding !

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