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JDK Flight Recorder vs. Stop the World Pauses

JDK Flight Recorder vs. Stop the World Pauses

Published June 9, 2021

BellSoft Blog Disclaimer

This post will focus on Stop-the-World pauses in the JVM and how JDK Flight Recorder (JFR) can help you identify the root cause. I’ve touched on this topic before in Hunting down memory issues…, but now the goal is to get a deeper insight into its aspects.


  1. Stop-the-World in HotSpot JVM
  2. Safepoints in JDK Flight Recorder
  3. Entering the safepoint
  4. Why may it take a long time to enter a safepoint?
    1. Thrashing
    2. Memory-mapped I/O
    3. Long internals between safepoint checks in JIT-compiled code
  5. Monitoring safepoints without JFR
  6. Conclusion

JFR is indeed a hidden gem of OpenJDK. This powerful tool is integrated in Liberica JDK 11 and newer. You can check it out and discover its use for your projects like I did for mine.

Stop-the-World in HotSpot JVM

Before moving on to details, I need to make some clarifications on terminology. The Stop-the-World (STW) pause is a general term for a state of the JVM when all application threads are suspended for the duration of a specific internal JVM operation.

In the HotSpot JVM used in OpenJDK, there is a more specific term: safepoint.

Citing the HotSpot Glossary, a safepoint is defined as:

A point during program execution at which all GC roots are known and all heap object contents are consistent. From a global point of view, all threads must block at a safepoint before the GC can run. (As a special case, threads running JNI code can continue to run, because they use only handles. During a safepoint they must block instead of loading the contents of the handle.) From a local point of view, a safepoint is a distinguished point in a block of code where the executing thread may block for the GC. Most call sites qualify as safepoints. There are strong invariants which hold true at every safepoint, which may be disregarded at non-safepoints. Both compiled Java code and C/C++ code be optimized between safepoints, but less so across safepoints. The JIT compiler emits a GC map at each safepoint. C/C++ code in the VM uses stylized macro-based conventions (e.g., TRAPS) to mark potential safepoints.

Safepoints are utilized not only for GC but also for other JVM operations. Below are key categories of JVM operations causing safepoins:

  • Garbage collector related;
  • JIT compiler related;
  • Bias locking related;
  • Diagnostic related (e.g., thread dump, heap dump, profiling, and debugging).

Besides explicit VM operations, certain cleanup tasks related to internal JVM housekeeping are delayed until the next safepoint. As there is no guarantee that a VM operation will run regularly, safepoint could be initiated without any explicit VM operation.

Effective pause time associated with a safepoint can be split into three components:

  • Entering safepoint time — the JVM needs all application threads to arrive at a safepoint, which does not happen instantly. This period is unrelated to the type of operation to be executed at a safepoint;
  • Executing tasks pending for the next safepoint;
  • VM operation time — operations obviously need some time to complete. This component is operation dependent.

Safepoints in JDK Flight Recorder

JDK flight recorder has multiple events related to safepoints, yet some are disabled by default. You can enable a safepoint-specific event in the “Start Flight Recording” dialog. Switch to low-level configuration mode, type “safepoint” in the filter, and check “Enabled” next to all options.

Start Flight Recording

If you have never started Flight Recorder before, detailed instructions are provided in my first post on JFR.

Once a recording is done, you can find the following views with information related to safepoints.

The standard report available in Mission Control is “VM Operations” under the “JVM Internals” category.

VM Operations” under the “JVM Internals

This report presents a summary of VM operations grouped by type. It is also possible to see individual events in the “Event log” tab.

Keep in mind that event durations in this report are just durations of VM operations and do not reflect the duration of the Stop-the-World pause.

Other safepoint-related events don’t have predefined reports in Mission Control so far. We will use “Event Browser” to view them. You can enter “safepoint” into the filter on top of the event type tree to see relevant events.

All event types in the screenshot below are disabled by default, so you need to enable them explicitly before starting Flight Recorder.

Flight Recorder

The “Safepoint Begin” event is the one stirring the most interest: its duration is the total duration of the Stop-the-World pause.

Information about safepoint is scattered among multiple events. Regardless, different events related to the same safepoint could be correlated by the “Safepoint Identifier” column.

We can use custom filters to make digging through safepoint details more ergonomic.

Right-click on the “Event Browser” node at the “Outline” side view on the left and choose “New Page” > “Custom Page …”, then select the “Safepoint Begin” event type to create a new custom report.

A new node will appear in the “Outline” side view; select it. You will see a report showing only “Safepoint Begin” events. Now, right-click and delete the existing filter node in the upper section of the report. Instead of filtering events by type, we will sieve all events with the “Safepoint Identifier” attribute. Right-click and choose “Add Filter from Attribute” > “Safepoint Identifier.” Lastly, change the type of the created filter node from “==” to “exists”.

Event Browser

The event log (section at the bottom) contains all safepoint-related events, including the “VM Operation” event. Now sort by “Start Time,” and all events associated with the same safepoint will be naturally grouped.

VM Operation

The diagram below shows relations between different JFR events affiliated with a safepoint.

JFR events

Normally the VM operation is the main factor defining the duration of the STW pause. Unfortunately, though, sometimes phases before the operation turn out to be problematic.

Entering the safepoint

Entering the safepoint is a fairly sophisticated process. The JVM needs to stop every Java thread at the point where all local variables are predictably stored at stack (many VM operations need to traverse stack for heap references). A cooperative safepoint protocol is used to coordinate running threads to enter a safepoint state.

Generally, each Java thread could be in one of the following conditions:

  • A thread is blocked with API in the runtime library (it’s not in the RUNNABLE state). In this case, no additional actions are required;
  • A thread is RUNNABLE and is running in the interpreter. In this case, the thread could be suspended on the next bytecode operation. The JVM replaces the interpreter jump table with special jump table variants where each byte code index is mapped to enter a safepoint routine;
  • A thread is RUNNABLE and is running JIT-compiled code. This is most tricky; the compiler could do a lot of operation reordering and load/store optimizations. The compiler has to emit special safepoint checks. The thread will suspend itself once it reaches the next checkpoint after the safepoint protocol is initiated by the JVM (I will get back to this topic later in the article);
  • A thread is RUNNABLE and is running native code via JNI. Such threads don’t need to be stopped, and they may continue to run during the JVM Stop-the-World pause. However, if this kind of a thread returns to calling Java method, calls Java method, or tries to access data in Java heap via JNI API, it will be blocked until the safepoint ends;
  • A thread is RUNNABLE and is running JVM intrinsic code or the “JavaCritical” JNI method. Such thread cannot be stopped until it will complete the current operation and enter the JIT-compiled code section.

The key thing is that all application threads need to be stopped. Until each and every thread enters a safepoint, the operation cannot start. Or put it another way—even if just one thread is stuck and isn’t able to enter for some time, it will effectively freeze the JVM.

Why may it take a long time to enter a safepoint?

All Java threads in the RUNNABLE state should enter a safepoint. Although RUNNABLE, in JVM terms, doesn’t necessarily mean that a thread is actually running on CPU at the moment.

The thread being RUNNABLE in Java could be:

  1. Actually running on a CPU;
  2. Scheduled to run by the OS but waiting for a CPU slice;
  3. Blocked due to blocking the OS call;
  4. Blocked due to a page fault.

Case 1 is the normal condition. The thread can proceed with the safepoint protocol.

Case 2 is possible if the host is starved of CPU resources. In Linux, it’s hard to monitor how long threads spend in the scheduler queue. Therefore, we can rely only on indirect metrics like total host CPU usage for post mortem diagnostics for such conditions. Remember that CPU starvation means overall degradation of performance, not just slowed-down VM operations.

Case 3 means the thread is blocked on an explicit OS call, which will be wrapped via JNI so that such thread will be exempt from the safepoint protocol.

Case 4 is a different story. Unlike Case 3, a page fault may block execution at any instruction. The JVM cannot add any safety net for these cases. The thread has to be resumed by the kernel and continue execution to, first, finish the safepoint protocol and then let it proceed with the VM operation.

It is obvious that Case 4 is a most problematic condition here, having the potential to cause abnormally long safepoint time.

What are common reasons for page faults in Java applications?


System-wide swapping (aka thrashing) could be one reason. As a rule of thumb, the JVM should keep its heap and a few other vital data structures resident in memory. If JVM’s memory starts to be paged out, it hits app performance quite badly. The GC also does not like memory being swapped out, so this condition has the double impact of the STW pauses.

Swapping, in turn, has two causes:

  • Host memory shortage — physical memory is not enough for all processes running on the host.
  • Container resident memory limit — it is possible to configure resident memory usage limit per container. This limit may lead to thrashing for a specific container even if the host has enough free memory (my previous post provides more details for this situation).

Memory-mapped I/O

Java runtime library has an API for memory-mapped disk I/O. “Memory-mapped” means that reading or writing to a file on disk are regular memory operations in application code. The actual content is loaded from disk to memory page on first access and eventually synchronized back to disk if modified. The kernel is responsible for moving data. It’s worth pointing out that for memory-mapped I/O moderate page fault rate is expected.

Just one thread busy with memory-mapped I/O is enough to impact the safepoint for the JVM globally.

Ironically, a memory mapping approach should reduce I/O overhead by eliminating OS calls, but in the JVM case, it can introduce worse problems interfering with the safepoint protocol.

I would advise avoiding memory-mapped I/O for services that are response time critical and process requests in parallel. Often libraries (e.g., Lucene) support both memory-mapped and non-memory-mapped I/O. You really need to assess the impact of the JVM STW pause before enabling the memory-mapped options in your middleware.

Long intervals between safepoint checks in JIT-compiled code

Above, I have described situations where Java threads are prevented from running by the OS. However, it’s possible for threads to run on CPU without yet completing the safepoint protocol in a timely manner.

The issue could be with the code produced by the JIT compiler. It should place checks for safepoint regularly in the emitted code.

Each method call site is typically eligible for a safepoint check (a method should always ensure its local variable is appropriately placed on the stack before calling another method).

Loop back branch is another natural point for a safepoint check, even though a safepoint implies a barrier in operation reordering, thus forcing many optimizations. A check for the safepoint may be omitted for “counted” loops (loops with an index variable of type int). The compiler takes risk for potentially long periods between checks in runtime and better general execution performance.

Besides the “counted” loop, JVM intrinsics can also have high upper bounds for execution time. For instance, when you create a huge array instance, array memory is zeroed at the moment of allocation with no safepoint possible until it’s done.

Fortunately, such situations are rare enough. The JIT compiler is well balanced in the JVM, and chances to hit this problem class is close to zero in typical Java code.

In my practice, there were just a handful of situations where I witnessed such a problem:

  • Operations on unreasonably long strings;
  • Extensive Java BigNumber manipulation;
  • Handmade statistics library working on long arrays of samples.

The problem has a further unpleasant side: safepoints are occurring fairly frequently in the JVM with a possibility of each becoming an unreasonably long STW pause.

The good news is that code profiling will typically be a highly problematic section of code as hot thanks to the safepoint bias.

Monitoring safepoints without JFR

JDK Flight Recorder is not the only tool to monitor safepoints. The JVM has multiple options to enable logging safepoint-related information to the console.

-XX:+PrintGCApplicationStoppedTime — contrary to its name, this option will let the JVM log all STW pauses, related or unrelated to garbage collection, to GC logs. If enabled, you’ll see lines like this:

Total time for which application threads were stopped: 0.0007555 seconds, Stopping threads took: 0.0005893 seconds

The message above does not tell you much about the nature of the safepoint, though.

-XX:+PrintSafepointStatistics -XX:PrintSafepointStatisticsCount=1 — such a combination of flags will let the JVM print more details after each safepoint. The output will look as follows:

         vmop     	[threads: total initially_running wait_to_block]    [time: spin block sync cleanup vmop] page_trap_count
9.220: ThreadDump 	[          17          8              8        ]    [       1	19    21     0     0  ]  8  

This output contains much more details, including VM operation, thread counts, and time (in milliseconds) spent in the pre-safepoint phases. It also contains the number of page traps during safepoint, which is very handy to identify I/O-related issues.

A downside here is formatting, weird and hard to parse. The idea behind this option is to log a summary of passing safepoints in the table form.

JDK 11 made available new structures logging system, and safepoint logging could be enabled with the following option: -Xlog:safepoint*=debug

The result will look like this:

[13.173s][debug][safepoint    	] Safepoint synchronization initiated. (18 threads)
[13.173s][info ][safepoint    	] Application time: 2.0725501 seconds
[13.173s][debug][safepoint    	] ... found polling page loop exception at pc = 0x0000000012975dbc, stub =0x000000000aed7100
[13.173s][debug][safepoint    	] ... found polling page loop exception at pc = 0x0000000012975dbc, stub =0x000000000aed7100
[13.173s][debug][safepoint    	] ... found polling page loop exception at pc = 0x0000000012975dbc, stub =0x000000000aed7100
[13.173s][debug][safepoint    	] ... found polling page loop exception at pc = 0x0000000012975dbc, stub =0x000000000aed7100
[13.173s][debug][safepoint    	] ... found polling page loop exception at pc = 0x0000000012975dbc, stub =0x000000000aed7100
[13.173s][debug][safepoint    	] ... found polling page loop exception at pc = 0x0000000012975dbc, stub =0x000000000aed7100
[13.173s][debug][safepoint    	] ... found polling page loop exception at pc = 0x0000000012975dbc, stub =0x000000000aed7100
[13.209s][debug][safepoint    	] ... found polling page loop exception at pc = 0x0000000012975dbc, stub =0x000000000aed7100
[13.209s][debug][safepoint    	] Waiting for 8 thread(s) to block
[13.210s][info ][safepoint    	] Entering safepoint region: ThreadDump
[13.210s][info ][safepoint,cleanup] deflating idle monitors, 0.0000004 secs
[13.210s][info ][safepoint,cleanup] compilation policy safepoint handler, 0.0000004 secs
[13.210s][info ][safepoint,cleanup] updating inline caches, 0.0000004 secs
[13.210s][info ][safepoint,cleanup] resizing system dictionaries, 0.0000004 secs
[13.210s][info ][safepoint,cleanup] purging class loader data graph, 0.0000000 secs
[13.210s][info ][safepoint,cleanup] safepoint cleanup tasks, 0.0000809 secs
[13.210s][info ][safepoint    	] Leaving safepoint region
[13.210s][info ][safepoint    	] Total time for which application threads were stopped: 0.0375086 seconds, Stopping threads took: 0.0372714 seconds


Stop-the-World (aka safepoints) pauses are fairly frequent in the JVM, and not all of them are related to garbage collection. Normally non-GC safepoints have submillisecond duration and go unnoticed.

And still, there are conditions that cause abnormal STW pauses due to a long time to safepoint.

Built-in JDK Flight Recorder profiles do not have detailed safepoint-related events disabled. If you suspect a non-GC-related STW pause, enabling this event in Flight Recorder is the first step in your investigation. Besides metrics collected by JFR, I also recommend looking at OS metrics to identify situations such as thrashing and CPU starvation.

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Alexey Ragozin

Java Expert, Special for BellSoft



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