Hunting down memory issues 
with JDK Flight Recorder

Hunting down memory issues with JDK Flight Recorder

Sep 2, 2020
Alexey Ragozin

In this post about JDK Flight Recorder and Mission Control, a powerful diagnostic and profiling tool built into OpenJDK, I would like to focus on JVM memory features.

What are the memory problems?

If you put the words “garbage collection” and “problem” into one sentence, many would think of the infamous Stop-the-World pauses that JVM is prone to trigger. Lengthy Stop-the-World pauses are indeed very problematic from an application perspective.

While garbage collection (GC) and Stop-the-World (STW) pauses are connected, they are not the same. The problem with GC is likely to manifest itself as prolonged STW pauses, but GC does not necessarily cause them.

General optimization of application code allocation patterns is another standard task, and Flight Recorder with Mission Control could be of great help here. Optimized memory allocation in application code suits both for GC and overall execution performance.

Finally, the application code may have memory leaks. Though I believe heap dumps are the best tool for diagnosing memory leaks, JDK Flight Recorder offers an alternative approach with its pros and cons. In particular, heap dumps require Stop-the-World heap inspection to capture and tend to be extensive (depending on the heap size), while JFR files do not grow proportionally to the heap size.

Below is a plan for the rest of this article.

  • Garbage collection related reports
  • JVM Operations aka the Stop-the-World pause report
  • Memory allocation sampling
  • Old object sampling overview

Starting memory profiling JDK Flight Recorder session

You would need OpenJDK with JDK Flight Recorder support and Mission Control. I was using Liberica OpenJDK 11.0.7+10 and Liberica Mission Control 7.1.1.

There are multiple ways to start a JDK Flight Recorder session. You can do it with Mission Control UI (locally or via JMX), with jcmd from console, or even configure it to auto-start with JVM command-line arguments.

All these options are covered in my previous post, so I will omit the details here and jump right to Flight Recorder session settings.

The configuration dialog of the Flight Recorder session in Mission Control has a few options relevant to the features I am going to explain later.

Garbage Collector

The default level is “Normal,” and it should be enough. You may choose “All,” which adds more details about GC Phases.

Memory Profiling

Here you may choose one of three options:

  • Off
  • Object Allocation and Promotion
  • All, including Heap Statistics

I strongly suggest you stay with “Object Allocation and Promotion.”

It provides valuable data points for allocation profiling with low overhead.

Heap Statistics included in the last option force regular Stop-the-World heap inspections which could be quite lengthy.

Memory Leak Detection

If you suspect memory leaks in your application, tweak these options. You need to choose the “Object Types + Allocation Stack Traces + Path to GC Root” level of details to see the leaking path in the report afterward. Calculating “Path to GC Root” is a moderately expensive Stop-the-World operation, and I would not recommend enabling it just as a precaution.

Garbage collection in OpenJDK

OpenJDK JVM can use different implementations of garbage collectors.

The most commonly used ones in HotSpot JVM are

  • Garbage First
  • Parallel Mark Sweep Compact
  • Concurrent Mark Sweep (CMS) — discontinued in Java 14

There are several other algorithms available in various JDKs, but I am not going to get too deep for the sake of brevity.

Each GC has its unique features, but a few are the same.

All the algorithms above are generational, meaning they split available heap space as separate young and old space. The result is three types of GC cycle:

  • Minor (young) collections — recycle only young space
  • Major (old) collections — recycle old and young space
  • Last resort full collections — a big STW whole heap collection you want to avoid

Each GC cycle is split into a hierarchy of phases. Some could be concurrent (executing in parallel with application code), but most require a Stop-the-World pause.

Garbage collector report

Mission Control has a generic “Garbage Collections” report agnostic of the GC algorithm used by JVM.

This report shows a list of GC pauses with break down by phases (table on the right).

Information in this report could replace the typical usage of GC logs. Here you can quickly spot the longest GC pause and dig a little deeper into its details. Notice GC ID assigned to each GC cycle: they are useful to pick additional events related to GC but not included in this report.

Two key metrics could be used to assess GC performance regardless of the algorithm:

  • Max duration of a Stop-the-World pause — it may add up to your application’s request processing time, hence increasing response time.
  • Percentage of time spent in a Stop-the-World pause during a certain period — e.g., if we spent 6 seconds per minute in GC, the application can utilize 90% of available CPU resources at most.

The chart on the “Garbage collection” report helps to spot both metrics easily. “Longest pause” and “Sum of pauses” visualize these metrics aggregated over regular time buckets (2 minutes in the screenshot).

“Longest pause” shows the longest GC pause in the bucket, so you can observe your regular long pauses and outliers, if applicable.

“Sum of pauses” shows the sum of all GC pauses in the bucket. Use this metric to derive the percentage of time spent on GC pauses. Hover your mouse over the bar, and you will see the exact numbers in the tooltip.

In the example screenshot, the sum of pauses over 2 minutes equals 4 seconds. GC overhead is 4 / 120 ≈ 3.3%, which is reasonable.

What if you are not happy with your GC performance?

GC tuning is a big topic out of scope for this article, but I will highlight a few typical cases.

‘Heap is too small’ problem

GC in modern JVM usually just works. A lot of engineering effort has gone into making GC adaptable for the application needs. Although GC is not magic, it needs enough memory to accommodate application live data sets and some head room to work efficiently.

If the heap size is not enough, you will see increased GC overhead (percentage of time spent on GC pauses) and, as a consequence, the application slowing down.

The screenshot above illustrates JVM running on low memory.

During the hovered time bucket, 1.4 seconds out of 2, JVM was in a GC pause, which is an absolutely unhealthy proportion.

Often increasing the max heap allowed to JVM solves the problem with high GC overhead. However, if an application has a memory leak, increasing heap size is no help. The leak in such a situation needs to be fixed.

‘Metaspace is too small’ problem

Metaspace is a memory area to store class metadata. Starting from Java SE 8, metaspace has not been part of heap, even though the lack of available space in metaspace may cause GC there.

To free some memory in metaspace, JVM needs to unload unused classes. Class unloading, in turn, requires a major GC on the heap. The reason is that objects on the heap keep references to corresponding classes, and a class cannot be unloaded until any instance of it exists in the heap.

You would quickly spot the metaspace keyword in the “Cause” column in this case. You can also view the metaspace size in the timeline chart at the bottom of the report to check if you are hitting the limit.

Special reference abuse

Java™ offers utility classes that have very specific semantics related to garbage collection. These are weak, soft and phantom references. Besides, JVM is using finalizer references internally to implement the semantics of finalizers.

All these references require particular processing within the bounds of a GC pause. Due to semantic special references, they can only be processed after all live objects are found.

Usually, this processing runs fast, but abusing these special references (especially finalizers as they are most expensive) could cause abnormally long pauses for otherwise healthy GC.

Columns with a count of each special reference type are on the right side in the GC event table. Numbers on the order of tens of thousands could indicate a problem.

You can also find the reference processing phase in the “Pause Phases” table and see how much time was spent on it.

The “Garbage Collections” report is a handy tool to spot standard problems, but it does not display all GC details available from JDK Flight Recorder.

As usual, you can get to the event browser and look at specific events directly. Look under Java Virtual Machine / GC / Details for more diagnostic data related to garbage collection.

Besides work done under Stop-the-World, some collectors (G1 in particular) use background threads working in parallel with application threads to assist in memory reclamation. You may find details for such tasks if you look at the “GC Phase Concurrent” event type.

Stop-the-World pauses in OpenJDK

A Stop-the-World (STW) pause is a state of JVM when all application threads are frozen, and internal JVM routines have exclusive access to the process memory.

Hotspot JVM has a protocol called safepoint to ensure proper Stop-the-World pauses.

STW pauses are mainly associated with GC activity, but it is but one possible reason. In Hotspot JVM, STW pauses get involved in other special JVM operations.


  • JIT compiler related operations (e.g. deoptimization or OSR)
  • Bias lock revocation
  • Thread dumps and other diagnostic operations, including JFR-specific ones

As a rule, STW pauses are unnoticeably short (less than a millisecond), but putting JVM on a safepoint is a sophisticated process. Things can go wrong here.

In the previous post, I mentioned the safepoint bias. Safepoint protocol requires each thread executing Java code to explicitly put itself in a “safe” state, where JVM knows exactly how local variables are stored in the memory on the stack.

After it starts, each thread running Java code receives a signal to put itself on the safepoint. A JVM operation cannot begin until all threads have confirmed their safe state (threads executing native code via JNI are an exception; STW does not stop them unless they try to access heap to switch from native code to Java).

If one or more threads are slow at reaching the safe state, the rest of the JVM will wait for them to be frozen effectively.

There are two main reasons for such misbehavior:

  • A thread is blocked while accessing a memory page (d state in Linux) and cannot react
  • A thread is stuck in a hot loop, where the JIT compiler omitted a safepoint check as it was considered too fast of a loop.

The first situation may happen if the system is swapping or application code is using memory-mapped IO. The second one may occur in heavily optimized computation-intensive code (in Java runtime, java.lang.Sting and java.util.BigDecimal operations over large objects are common culprits).

VM Operations report

Mission Control has a report with a summary of all Stop-The-World VM operations.

This report shows a summary grouped by type of operations. In the screenshot, “CGC_Operation” and “G1CollectForAllocation” are the only operations related to GC (G1 in particular).

You can see a fair number of other VM operations, but they are very quick.

Also, bear in mind that many operations are caused by JFR itself, such as “JFRCheckpoint,” “JFROldObject,” “FindDeadlocks,” etc.

Problems with safepoints are relatively rare in practice, but spending a few seconds to quickly check the “VM Operations” report for abnormal pauses is a good habit.

Memory allocation profiling

While JVM is heavily optimized for allocating a lot of short-lived objects, excessive memory allocation may cause various problems.

Allocation profiling helps to identify which code is responsible for the most intense allocation of new objects.

Mission control has a “Memory” report giving an overview of object allocation in application code.

This report shows a timeline of the application’s heap allocation rate and a histogram of allocation size by object type.

At first glance, the report looks too ascetic and done in broad strokes, notably if you have used Mission Control 5.5.

There are many more details to squeeze out from this report with Mission Control magic, but we need to get an idea of the source for these numbers.

How are allocation profiling data collected?

Allocation profiling in JVM was a rather challenging task. While previously many profilers had allocation profiling features, the performance impact on an application with allocation profiling turned on was inconvenient and often prohibitive.

Allocation profiling is based on sampling. A runtime allocation profiler collects a sample of allocation events to get the whole picture. Before JEP-331 (available since OpenJDK 11), profilers had to instrument all Java code and inject extra logic at every allocation site. The performance impact from such code mangling was dramatic.

JDK Flight Recorder, on the contrary, was always able to use low overhead allocation profiling. The key is to record TLAB allocation events instead of sampling normal ones.

Almost all new Java objects are allocated in the so-called Eden (a part of young object heap space). Not to compete for shared memory management structures, each Java thread reserves a thread local allocation buffer (TLAB) in Eden and allocates new objects there. It is an essential performance optimization for multicore hardware.

Eventually, the buffer dries up, and the thread has to allocate a new one. This type of event is the one recorded by Flight Recorder. The average TLAB size is roughly 500KiB (size is dynamic and changes per thread), so one allocation per ~500 KiB of allocated memory is recorded. This way, Flight Recorder does not add any overhead to the hot path of object allocation, yet it can get enough samples for further analysis.

Each sample has a reference to the Java thread, stack trace, type and size of the object being allocated, as well as the size of the new TLAB. You can find raw events in the event browser under “Allocation in new TLAB.”

But how would I know which code is allocating the most?

The “Memory” report is based on the “Allocation in new TLAB” events, which has associated thread and stack trace. It means we can use the “Stack Trace” view and filtering to extract much more details from this report.

Typical workflow for identifying memory hot spot would look as follows:

  • Open the “Memory” report.
  • Open the “Stack Trace” view (enable via Window > Show View > Stack Trace if hidden).
  • Enable “Show as Tree” and “Group traces from last method” on the “Stack Trace” view. Such a configuration is the most convenient for this kind of sampling.
  • Switch “Distinguish Frames By” to “Line Number” in the context menu of the “Stack Trace” view if you want to see line numbers.
  • By this point, you can already see a top hot spot of allocation in your application. You may also select a specific object type in the “Memory” report to only examine these type-related allocation traces.

Further, you can select a range of timelines to zoom in on a definite period. Another functionality here is applying filters to narrow the report to individual threads. All the tricks for the “Method Profiling” report described in the previous post would work here too.

Live object sampling

Memory leaks are another well-known problem for Java applications. Traditional approaches to memory leak diagnostics rely on heap dumps.

While I consider heap dumps a vital tool for dealing with memory troubleshooting optimizations, they have their drawbacks. More specifically, heap dumps could be quite large, slow to process and require Stop-the-World heap inspection to capture.

Flight Recorders offer an alternative approach, live object sampling.

The idea is simple: we already have allocation sampling. Now from the newly allocated objects, Flight Recorder picks few to trace across their lifetime. If an object dies, it is excluded from the sample (and no event is produced).

Then events are dumped into a file at the end of recording. Flight Recorder may optionally calculate and record the path to the nearest GC root for each object in a live object sample (a configuration option mentioned at the beginning of this article). This operation is expensive—requiring an STW heap inspection—but necessary to understand how an object is leaking.

But these are just random objects. How would sampling help find a leak?

Normal objects are short-lived, but leaked objects could survive for long. If sampling JVM for a long enough time, you will likely catch a few leaked objects in the sample. Once these objects get to the final report, we can identify where they are leaking via the path to the nearest GC root, also recorded.

Mission Control has a “Live Objects” report to visualize this kind of event. You would also like the “Stack Trace” view to be enabled here.

In the table, you can see a sampled object grouped by GC root. You can unfold a reference path from a GC root down to individual objects from the sample.

What is great is that you can see the allocation stack trace of each sampled object in the “Stack Trace” view as well. This information is unique to Flight Recorder and cannot be reconstructed from a regular heap dump.

Does it work?

This feature is relatively new in Flight Recorder, whereas heap dump analysis is a reliable, time-tested technique.

A live object sample in Flight Recorder is rather small, and it is a matter of luck whether it would catch a problematic object or not. On the other hand, Flight Recorder files several orders smaller in magnitude compared to heap dumps.

While Old Object Sampling is definitely not a silver bullet, it offers a fascinating alternative to the traditional heap dump wrangling approach for memory leak investigations.


Memory in JVM is a complicated matter. JDK Flight Recorder and Mission Control have many features to help users with typical problems. Nonetheless, the learning curve remains steep.

On the bright side, complex JVM machinery such as GC and safepoints usually works fine on its own, so there is rarely a need to dig deep. Still, it is wise to know where to look if you have to.

Memory optimization is a regular task. The allocation profiling features in Flight Recorder and Mission Control are highly beneficial in practice. If you asked me about the single most crucial Flight Recorder component, I would name allocation profiling without a second thought.

Live object sampling is also an appealing approach for memory leak detection. Unlike heap dumps, it is much more appropriate for automation and continuous usage in production environments.

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