Allocation Stalls in Java ZGC: Causes and Solutions

In ZGC, a type of concurrent Garbage Collection (GC) algorithm, GC threads run concurrently with application threads, resulting in minimal stop-the-world pauses. However, because these pauses are so brief, application threads may create objects faster than GC threads can reclaim memory. In such cases, the JVM temporarily stops the application threads from creating new objects. This 'stopping of object creation' is known as an "Allocation Stall."

What Causes Allocation Stall?

Allocation Stall occurs due to following reasons:

1. Inefficient GC Algorithm: This is often the primary cause of Allocation Stall. Using a non-optimal GC algorithm or improper GC settings for your application's workload can lead to stalling. Earlier versions of ZGC (i.e. single-generation ZGC algorithm), are more prone to Allocation Stalls.
2. High Object Allocation Rate: If your application creates objects at a very high rate, it can overwhelm the GC's ability to reclaim memory quickly enough, leading to stalls.
3. Memory Fragmentation: Even if there is free memory, fragmentation in the heap can prevent large objects from being allocated, contributing to Allocation Stalls.

Symptoms of Allocation Stall

If your application suffers from Allocation Stall, it may exhibit the following symptoms:

1. Application Becomes Unresponsive or Reduced Throughput: One of the most noticeable symptoms is intermittent unresponsiveness or a general degradation in throughput. Users may experience slow or delayed responses as the application spends more time managing memory rather than executing business logic.
2. Increased Latency or Hiccups: Requests that normally process quickly may start taking longer, or the application may experience sporadic pauses in its ability to handle requests.
3. Long GC Pauses: Allocation Stalls often coincide with frequent or prolonged garbage collection cycles as the GC struggles to free up memory fast enough to meet the application's allocation needs.
4. CPU Spikes: During Allocation Stalls, you may observe CPU spikes as the GC threads work hard to reclaim memory, potentially preempting the application threads and contributing to overall system slowdowns.
5. Error Messages Related to Memory: Depending on the severity, your application may encounter OutOfMemoryError exceptions, especially if Allocation Stall conditions persist and the GC cannot reclaim enough memory.

Allocation Stall Solutions

When Allocation Stalls happens in your application, below are the potential solutions to address them:

1. Switch to Generational ZGC (JDK 21+)

Allocation Stalls were quite common in the earlier single-generation ZGC algorithm. However, in JDK 21, a Generational ZGC algorithm was introduced, which divides the JVM heap into two compartments: the Young Generation and the Old Generation.

Young Generation:This compartment is used to allocate short-lived objects, reducing the need for frequent full heap scans.

Old Generation:This stores long-lived objects, minimizing the work the GC has to do on each cycle, leading to fewer Allocation Stalls.

This generational approach helps the GC operate more efficiently, significantly reducing Allocation Stalls. Learn more about Generational ZGC algorithm.

To enable Generational ZGC in JDK 21 or later, use the following JVM argument:

<<start:code>>
-XX:+UseZGC -XX:+ZGenerational
<<end:code>>

2. Increase Max Heap Size

If the heap is too small for your application's memory requirements, it can quickly run out of space, causing Allocation Stalls. Increasing the maximum heap size (-Xmx argument) allows your application to allocate more memory before triggering GC events, potentially reducing stalls.

To learn how to calculate the right heap size for your application, check out this detailed guide on optimizing memory allocation.

3. Increase GC Threads

Allocating more GC threads can help process memory reclamation faster, especially in high-throughput environments. However, it's important to balance the number of GC threads, as having too many can pre-empt application threads, slowing down the overall performance.

To learn how to adjust the number of GC threads and the appropriate JVM flags, refer to this detailed guide on optimizing memory allocation.

4. Reduce Live Data or Allocation Rate

Review your application's memory usage to reduce the amount of live data and the allocation rate. Some strategies include:

1. Reusing objects instead of frequently creating new ones.
2. Employing object pools to manage expensive-to-create objects.
3. Reducing the size and scope of temporary objects.

5. Use Large Pages

Passing the -XX:+UseLargePages JVM argument can help improve memory management by reducing the number of memory pages the JVM has to handle. Large pages provide more contiguous memory, reducing fragmentation and improving performance, which can mitigate Allocation Stalls.

For a detailed explanation on how large pages work and when to use them, check out this guide on using the -XX:+UseLargePages JVM argument.

6. Off-Heap Memory Usage

Consider using off-heap memory to reduce heap pressure. For large datasets or caches, consider using ByteBuffer APIs and offload memory management from the JVM heap, reducing the likelihood of Allocation Stalls.

Conclusion

Allocation Stall can have a significant impact on your application's responsiveness. By understanding the causes and implementing the right tuning strategies—such as switching to Generational ZGC, increasing heap size, optimizing GC threads, and managing memory more efficiently—you can greatly reduce the frequency and severity of these stalls. Tools like GCeasy can help you monitor and resolve these issues more effectively.