![]() One good example is the reloading of a large cache subsystem. ![]() The second effect is that AP protects the application from an unexpected high activity of some individual threads. For the application, this will look more like a throughput reduction than a sudden latency. The first and primary effect of AP is the spreading of long allocation delays into many smaller ones. There is no cost while heap usage is below the pacing threshold.Threads can not suddenly consume the whole heap.Significantly reduces the maximum magnitude of allocation delays.The GC completes a few cycles but isn’t able to reclaim enough memory.Īllocation delay happens if the heap usage exceeds the pacing threshold, which can result in two outcomes.ĪP allows allocation to proceed based on global allocation rate and the remaining space. The GC successfully reclaims enough memory. WITHOUT AP What Happens?Īllocation delay happens if the requested size doesn’t fit in the remaining free space, which can result in two outcomes. The reduction is equally proportional between the threads to the requested memory size, thus achieving a fair and proportional distribution of the pacing, without hiccups. The enforced rate is computed dynamically depending on the remaining free space, thus “smoothing” the trajectory leading to the Xmx boundary. AP enforces the maximum allocation rate to reach a better match with the GC cycle length. If the heap usage keeps growing and reaches the pacing threshold ( red line), the AP becomes active. This may lead the heap usage to go over the GC target ( green line). The application may surprise the GC with a higher allocation rate or live set than predicted. Depending on the rate of how memory gets allocated (climbing black line), a prediction is made to start the GC cycle ( green point) at the right time not to exceed the target. The command-line argument Xmx defines the memory limit assigned to the JVM ( grey line).īased on a configured value, a target is defined for the GC to keep the total used memory below a specific percentage of Xmx ( green line). Let’s explain Allocation Pacing based on this example graph. ZST coordinates memory use between Azul Platform Prime and the Linux operating system. AP can be disabled with -XX:-GPGCUseAllocationPacing. Giving GC more time to complete its collection helps to avoid exhausting heap space, which can lead to long stalls.ĪP is available only in non-ZST mode. AP helps to reduce peak allocation delays by limiting the allocation rate of the application when the heap usage approaches Xmx.ĪP does this by introducing many small delays into the allocation paths, proportional to the requested allocation size. If this happens, delays occur as memory allocations have to wait till the GC finishes.Īllocation Pacing (AP) is an additional technique within the JVM of Azul Zulu Prime builds of OpenJDK to prevent long memory allocation delays. ![]() The GC operates by cycles, and running a cycle takes some time.Īzul Zulu Prime uses the C4 Garbage Collector, which runs concurrently with your Java application.ĭuring the GC cycle, the application may outrun the GC and exhaust the memory before the GC completes. ![]() The Java Virtual Machine (JVM) that runs your Java applications has a Garbage Collector (GC) responsible for recycling memory objects that are no longer needed. Frank Delporte is a Java Champion, Java developer, technical writer at Azul, blogger, author of "Getting started with Java on Raspberry Pi", and contributor to Pi4J.
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