[jira] [Created] (ZOOKEEPER-2024) Major throughput improvement with mixed workloads

["Kfir Lev-Ari (JIRA)" <ji...@apache.org>]

Kfir Lev-Ari created ZOOKEEPER-2024:
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             Summary: Major throughput improvement with mixed workloads
                 Key: ZOOKEEPER-2024
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2024
             Project: ZooKeeper
          Issue Type: Improvement
          Components: quorum, server
            Reporter: Kfir Lev-Ari
         Attachments: ZOOKEEPER-2024.patch

The patch is applied to the commit processor, and solves two problems:

1. Stalling - once the commit processor encounters a local write request, it stalls local processing of all sessions until it receives a commit of that request from the leader. 
In mixed workloads, this severely hampers performance as it does not allow read-only sessions to proceed at faster speed than read-write ones.
2. Starvation - as long as there are read requests to process, older remote committed write requests are starved. 
This occurs due to a bug fix (https://issues.apache.org/jira/browse/ZOOKEEPER-1505) that forces processing of local read requests before handling any committed write. The problem is only manifested under high local read load. 

Our solution solves these two problems. It improves throughput in mixed workloads (in our tests, by up to 8x), and reduces latency, especially higher percentiles (i.e., slowest requests). 
The main idea is to separate sessions that inherently need to stall in order to enforce order semantics, from ones that do not need to stall. To this end, we add data structures for buffering and managing pending requests of stalled sessions; these requests are moved out of the critical path to these data structures, allowing continued processing of unaffected sessions. 
In order to avoid starvation, our solution prioritizes committed write requests over reads, and enforces fairness among read requests of sessions. 

Please see the docs:  
1) https://docs.google.com/document/d/1oXJiSt9VqL35hCYQRmFuC63ETd0F_g6uApzocgkFe3Y/edit?usp=sharing - includes a detailed description of the new commit processor algorithm.

2) The attached patch implements our solution, and a collection of related unit tests

3) https://docs.google.com/spreadsheets/d/1vmdfsq4WLr92BQO-CGcualE0KhAtjIu3bCaVwYajLo8/edit?usp=sharing - shows performance results of running system tests on the patched ZK using the patched system test from https://issues.apache.org/jira/browse/ZOOKEEPER-2023. 

See also https://issues.apache.org/jira/browse/ZOOKEEPER-1609




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