[jira] [Commented] (RATIS-2403) Improve linearizable follower read throughput instead of writes

["Ivan Andika (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/RATIS-2403?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18064984#comment-18064984 ] 

Ivan Andika commented on RATIS-2403:
------------------------------------

Found a paper that might be useful: [https://www.vldb.org/pvldb/vol18/p2831-giortamis.pdf]

It introduces Almost-Local Reads (ALRS) that have some kind of batching implementation, but seems to be batching reads on the follower instead of the batching writes on the leader (which is what we are pursuing). The paper claims it is linearizable with significant performance improvement (2.5x for Raft).

> Improve linearizable follower read throughput instead of writes
> ---------------------------------------------------------------
>
>                 Key: RATIS-2403
>                 URL: https://issues.apache.org/jira/browse/RATIS-2403
>             Project: Ratis
>          Issue Type: Improvement
>          Components: Linearizable Read
>            Reporter: Ivan Andika
>            Assignee: Ivan Andika
>            Priority: Major
>         Attachments: 1362_review.patch, leader-backpressure.patch, leader-batch-write.patch
>
>          Time Spent: 1h 20m
>  Remaining Estimate: 0h
>
> While benchmarking linearizable follower read, the observation is that the more requests go to the followers instead of the leader, the better write throughput becomes, we saw around 2-3x write throughput increase compared to the leader-only write and read (most likely due to less leader resource contention). However, the read throughput becomes worst than leader-only write and read  (some can be below 0.2x). Even with optimizations such as RATIS-2392 RATIS-2382 [https://github.com/apache/ratis/pull/1334] RATIS-2379, the read throughput remains worse than leader-only write (it even improves the write performance instead of the read performance).
> I suspect that because write throughput increase, the read index increases at a faster rate which causes follower linearizable read to wait longer.
> The target is to improve read throughput by 1.5x - 2x of the leader-only write and reads. Currently pure reads (no writes) performance improves read throughput up to 1.7x, but total follower read throughput is way below this target.
> Currently my ideas are
>  * Sacrificing writes for reads: Can we limit the write QPS so that read QPS can increase
>  ** From the benchmark, the read throughput only improves when write throughput is lower
>  ** We can try to use backpressure mechanism so that writes do not advance so quickly that read throughput suffer
>  *** Follower gap mechanisms (RATIS-1411), but this might cause leader to stall if follower down for a while (e.g. restarted), which violates the majority availability guarantee. It's also hard to know which value is optimal for different workloads.
> Raising this ticket for ideas. [~szetszwo] [~tanxinyu] 



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