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https://issues.apache.org/jira/browse/RATIS-2403?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18057643#comment-18057643
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Tsz-wo Sze commented on RATIS-2403:
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[~ivanandika], thanks for benchmarking and providing the interesting ideas for
improvement.
One thing we could try is to batch the writes -- instead of processing the
writes one by one, process them in batches. It is to make the appliedIndex
(i.e. the readIndex) change less frequently. For example,
- Processing one by one: appliedIndex increments by 1ms
- Processing in batches (time batching): appliedIndex increments every 10ms
Of course, we still want to keep the state machine busy. We will keep applying
the transaction one by one but reply to the clients in batches. So, we may
need one more index, repliedIndex, and use it as the readIndex (instead of
using appliedIndex).
> 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
> Reporter: Ivan Andika
> Priority: Major
>
> 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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