Hi dev, 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 (MicroRequestThread.java) 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%. ==== SELECT / sec ==== # of threads Batch 2ms Group 10ms 1 192 103 2 163 212 4 264 416 8 454 800 16 744 1311 32 1151 1481 64 1767 1844 128 2949 3011 256 4723 5000 ==== UPDATE / sec ==== # of threads Batch 2ms Group 10ms 1 45 26 2 39 51 4 58 102 8 102 198 16 167 213 32 289 295 64 544 548 128 1046 1058 256 2020 2061 Thanks, Yuji
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