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Spring Webflux performance issues in 1.57

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Description

agent-java community triage

Describe the bug

The new co.elastic.apm.agent.reactor.SubscriptionCancelInstrumentation has significant impact on performance in Spring Webflux project, especially when using alongside something that runs with low latency like Redis causing exceptions like this:

Unable to acquire connection! java.util.concurrent.CompletableFuture@18e0306a[Completed exceptionally: java.util.concurrent.CancellationException]Increase connection pool size. Node source: NodeSource [slot=0, addr=null, redisClient=null, redirect=null, entry=null], command: (EXPIRE), params: [[115, 112, 114, 105, 110, 103, 58, 115, 101, 115, ...], 1209900] after 1 of 2 retry attempts
at org.redisson.command.RedisExecutor.lambda$scheduleRetryTimeout$0(RedisExecutor.java:309)
at io.netty.util.HashedWheelTimer$HashedWheelTimeout.run(HashedWheelTimer.java:717)
at io.netty.util.concurrent.ImmediateExecutor.execute(ImmediateExecutor.java:34)
at io.netty.util.HashedWheelTimer$HashedWheelTimeout.expire(HashedWheelTimer.java:705)
at io.netty.util.HashedWheelTimer$HashedWheelBucket.expireTimeouts(HashedWheelTimer.java:791)
at io.netty.util.HashedWheelTimer$Worker.run(HashedWheelTimer.java:501)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.lang.Thread.run(Thread.java:1474)

I tested the throughput of the "non-instrumentation" solution that we discussed here and I saw ~10x improvement in throughput or latency of http calls of the service

Steps to reproduce

Steps to reproduce the behavior:

  1. Create Spring Boot 4 Webflux project with some simple API and use cache using Redis in it
  2. Compare throughput of the service http calls with and without APM client version 1.57

Expected behavior

Throughput should be very similar ( not 10x diff )

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