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From "Yuji Ito (JIRA)" <>
Subject [jira] [Created] (CASSANDRA-13530) GroupCommitLogService
Date Sat, 13 May 2017 12:57:04 GMT
Yuji Ito created CASSANDRA-13530:

             Summary: GroupCommitLogService
                 Key: CASSANDRA-13530
             Project: Cassandra
          Issue Type: Improvement
            Reporter: Yuji Ito
             Fix For: 2.2.x, 3.0.x, 3.11.x
         Attachments: groupCommit22.patch, groupCommit30.patch, groupCommit3x.patch,

I propose a new CommitLogService, GroupCommitLogService, to improve the throughput when lots
of requests are received.
It improved the throughput by maximum 94%.
I'd like to discuss about this CommitLogService.

Currently, we can select either 2 CommitLog services; Periodic and Batch.
In Periodic, we might lose some commit log which hasn't written to the disk.
In Batch, we can write commit log to the disk every time. The size of commit log to write
is too small (< 4KB). When high concurrency, these writes are gathered and persisted to
the disk at once. But, when insufficient concurrency, many small writes are issued and the
performance decreases due to the latency of the disk. Even if you use SSD, processes of many
IO commands decrease the performance.

GroupCommitLogService writes some commitlog to the disk at once.
The patch adds GroupCommitLogService (It is enabled by setting `commitlog_sync` and `commitlog_sync_group_window_in_ms`
in cassandra.yaml).
The difference from Batch is just only waiting for the semaphore.
By waiting for the semaphore, some writes for commit logs are executed at the same time.
In GroupCommitLogService, the latency becomes worse if the there is no concurrency.

I measured the performance with my microbench ( by increasing the
number of threads.The cluster has 3 nodes (Replication factor: 3). Each nodes is AWS EC2 m4.large
instance + 200IOPS io1 volume.
The result is as below. The GroupCommitLogService with 10ms window improved update with Paxos
by 94% and improved select with Paxos by 76%.

h6. SELECT / sec
||\# of threads||Batch 2ms||Group 10ms||

h6. UPDATE / sec
||\# of threads||Batch 2ms||Group 10ms||

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