Troubleshooting Jetty 12 LowResourceMonitor Issues A Comprehensive Guide

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Hey guys, we've been digging into some funky behavior with Jetty 12's LowResourceMonitor and wanted to share what we've found. Specifically, we've noticed cases where the monitor gets stuck in a lowOnResources=true state, even after the server isn't actually low on resources anymore. This can cause some serious headaches, so let's break down what we've observed, what might be causing it, and how we can tackle this issue.

Observations of the LowResourceMonitor Issue

Here are the key observations we've made while investigating this problem. We've seen this across different environments, so it seems like a pretty consistent issue. It's important to note these observations because they highlight the core of the problem and give us clues for where to look for solutions. Let's dive in!

First, we've confirmed the stuck state via JMX. This is crucial because it gives us a direct, real-time view into what the LowResourceMonitor is reporting.

  • We've seen that the LowResourceMonitor stubbornly shows lowOnResources = true. This is our primary symptom.
  • The listed Reasons often indicate Server low on threads: 100, idle threads: 0. This suggests the monitor initially triggered due to thread pool exhaustion.
  • Here's the kicker: the associated thread pool was not in a low-thread state at the time. It was happily reporting lowOnThreads = false. This mismatch is a major red flag, showing that the monitor isn't correctly reflecting the actual server status. This is a serious issue as it prevents the server from recovering properly.
  • Worse yet, the low-resource condition never cleared automatically. This means the server remained in a degraded state, potentially impacting performance and availability. We need the monitor to be dynamic and responsive to changes in the server's resource utilization.

Secondly, we've observed a potentially related exception in the logs from a different service running Jetty 12.0.18. This exception, while not directly tied to the stuck state on 12.0.22, gives us a vital clue about a possible underlying mechanism. This exception provides a critical piece of the puzzle.

java.util.concurrent.TimeoutException: Idle timeout expired: 185/100 ms
	at org.eclipse.jetty.io.IdleTimeout.checkIdleTimeout(IdleTimeout.java:167)
	Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
	*__checkpoint ⇢ com.opentable.server.reactive.webfilter.BackendInfoWebFilterConfiguration$BackendInfoWebFilter [DefaultWebFilterChain]
	*__checkpoint ⇢ com.opentable.conservedheaders.reactive.ConservedHeadersWebFilter [DefaultWebFilterChain]
	*__checkpoint ⇢ HTTP POST "/announcement" [ExceptionHandlingWebHandler]
Original Stack Trace:
		at org.eclipse.jetty.io.IdleTimeout.checkIdleTimeout(IdleTimeout.java:167)
		at org.eclipse.jetty.io.IdleTimeout.idleCheck(IdleTimeout.java:113)
		at org.eclipse.jetty.io.IdleTimeout.activate(IdleTimeout.java:136)
		at org.eclipse.jetty.io.IdleTimeout.setIdleTimeout(IdleTimeout.java:100)
		at org.eclipse.jetty.server.LowResourceMonitor.setLowResources(LowResourceMonitor.java:362)
		at org.eclipse.jetty.server.LowResourceMonitor.monitor(LowResourceMonitor.java:302)
		at org.eclipse.jetty.server.LowResourceMonitor$1.run(LowResourceMonitor.java:76)
		at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:572)
		at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
		at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
		at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
		at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
		at java.base/java.lang.Thread.run(Thread.java:1583)
  • This TimeoutException points to a potential issue within org.eclipse.jetty.io.IdleTimeout. Specifically, the stack trace shows it occurring during the checkIdleTimeout process.
  • Crucially, the stack trace implicates LowResourceMonitor.setLowResources() as the origin of the problem. This strongly suggests that the call to EndPoint.setIdleTimeout(...) within LowResourceMonitor.setLowResources() can throw an exception.
  • The most important implication is that this exception could interrupt the monitor thread itself. If the monitor thread is interrupted, it might fail to reset the lowOnResources flag, leading to the stuck state. This is our main hypothesis for what's going on.
  • It's worth noting that while this specific exception was observed on Jetty 12.0.18, the stuck LowResourceMonitor case happened on 12.0.22. This suggests the underlying issue might persist across versions, even if the exact manifestation differs. This gives us a wider scope to investigate, confirming that this can be a bug in different versions of Jetty.

Potential Root Cause: Interrupted Monitor Thread

Based on our observations, our leading hypothesis is that the LowResourceMonitor gets stuck due to an interrupted monitor thread. Let's break this down:

  1. The LowResourceMonitor uses a background thread to periodically check server resource levels (like thread pool usage). This is a common pattern for monitoring tasks.
  2. When resources are low, the monitor calls setLowResources(). This method likely performs several actions, including potentially adjusting idle timeouts on connections via EndPoint.setIdleTimeout(...). It’s responsible for setting the low resource flags.
  3. The EndPoint.setIdleTimeout(...) call seems to be the weak link. As the observed TimeoutException suggests, this call can throw an exception under certain circumstances. This is what we saw in the logs, so let's investigate further.
  4. If an exception is thrown during the execution of setLowResources(), especially within the monitor thread, it could interrupt the thread's normal execution flow. Think of it like tripping over a wire while walking – you might not complete your journey.
  5. The critical consequence: the monitor thread might fail to complete its full cycle, especially the part where it resets lowOnResources when the resource pressure eases. This is the core problem, the heart of why the monitor gets stuck.
  6. The result: the LowResourceMonitor remains stuck in the lowOnResources=true state, even when the thread pool recovers. This false positive can trigger unnecessary actions and degrade server performance. This is the main symptom we're seeing, and it's what we need to fix.

To further solidify this hypothesis, we can consider these points:

  • The intermittent nature of the issue: The exception might not be thrown every time, explaining why the monitor doesn't get stuck consistently. Timing and specific server load conditions might play a role.
  • The thread pool mismatch: The thread pool reporting lowOnThreads = false while the monitor says lowOnResources = true strongly supports the idea that the monitor's state is out of sync with reality. This is a key piece of evidence.

Steps to Reproduce the Issue

Reproducing the issue consistently is key to validating our hypothesis and developing a fix. While we haven't nailed down the exact steps for a 100% reliable reproduction, here's what we've gathered so far and what we can try:

  1. Simulate a Thread Pool Exhaustion: The initial trigger for the LowResourceMonitor seems to be thread pool exhaustion. We need to create a scenario where the server's thread pool gets saturated.
    • Load Testing: Bombard the server with a high volume of requests. This is the most straightforward approach, simulating heavy traffic.
    • Long-Running Tasks: Introduce tasks that consume threads for extended periods. This can artificially keep threads busy and prevent them from being released back to the pool. This helps to keep the pool busy.
    • Combination: A mix of both load testing and long-running tasks might be the most effective way to stress the thread pool.
  2. Monitor JMX: Keep a close eye on the LowResourceMonitor and the thread pool metrics via JMX. This will allow us to observe the lowOnResources flag and thread pool status in real-time. This is like keeping a close eye on the vital signs of our server.
  3. Look for the Stuck State: Once the LowResourceMonitor triggers (shows lowOnResources = true), observe if it clears automatically when the thread pool recovers. This is the critical point – does the monitor reset itself?
  4. Check Logs for Exceptions: Even if the monitor doesn't get stuck, thoroughly examine the logs for the java.util.concurrent.TimeoutException or any other exceptions related to IdleTimeout or LowResourceMonitor. This is like looking for clues in a mystery novel.

Specific things to try:

  • Vary Connection Idle Timeouts: Experiment with different settings for connection idle timeouts. This might influence the frequency of the TimeoutException.
  • Introduce Network Latency: Simulate network delays. This could exacerbate idle timeout issues and increase the likelihood of the exception.
  • Run Multiple Services: If possible, run multiple services on the same Jetty instance. This can increase resource contention and make the issue more likely to surface.

By systematically trying these steps and carefully monitoring the server, we should be able to create a reproducible scenario. This will be a huge step towards finding a definitive solution.

Potential Solutions and Workarounds

Okay, so we've identified the problem and have a good hypothesis about the root cause. Now, let's talk solutions! Here are some potential fixes and workarounds we can explore. It's important to consider both short-term mitigations and long-term solutions.

1. Patching Jetty (Ideal Long-Term Solution)

The most robust solution is to address the underlying bug in Jetty itself. This will require a code change within the LowResourceMonitor or related classes. Here's a potential approach:

  • Exception Handling: The key is to ensure that exceptions thrown during setLowResources() don't interrupt the monitor thread. We can achieve this by wrapping the potentially problematic code (specifically the call to EndPoint.setIdleTimeout(...)) in a try-catch block.
  • Thread Interruption Handling: Within the catch block, we need to log the exception (for debugging purposes) but prevent it from propagating and interrupting the thread. This might involve explicitly catching InterruptedException or using a finally block to ensure cleanup and resetting the monitor state.
  • Proper Monitor State Reset: We need to guarantee that the lowOnResources flag is reset when the resource situation improves, even if an exception occurred. This could involve adding explicit checks and resets within the catch or finally block.

This fix would involve modifying Jetty's source code and building a patched version. We would ideally contribute this patch back to the Jetty project so that it can be included in future releases. This is the most sustainable and robust approach.

2. Workaround: Manual Reset via JMX (Short-Term Mitigation)

As a temporary workaround, we can manually reset the lowOnResources flag via JMX. This won't fix the underlying problem, but it will allow us to recover from the stuck state without restarting the server.

  • Monitor JMX: We need to actively monitor the LowResourceMonitor via JMX.
  • Manual Reset: When we detect the stuck state (lowOnResources = true despite the thread pool being healthy), we can use JMX to manually set the flag to false. This will essentially tell the monitor,