In Java, GC roots can be four types of objects: Stop-The-World means that the execution of the program is suspended for GC till all objects in the heap are processed. GC on the Java virtual machine (JVM) follows the Stop-The-World mechanism. Moreover, through this process, objects that are dead or unused for a long time in the memory heap will be deleted and the memory space used by these objects will be reclaimed. Through performing the GC mechanism, available memory can be effectively used. 4.3.Garbage collection (GC), as its name implies, is a means of freeing space occupied by waste materials, or garbage, and avoid memory leaks. What this means is that the read and write must happen from the to-space that is guaranteed to survive the evacuation. Shenandoah manages to achieve this by performing a compare-and-swap operation on the Brooks pointer of an object to point to its to-space version:įurther, Shenandoah uses the read and write barriers to ensure that a strict “to-space” invariant is maintained during the concurrent evacuation. A concurrent relocation of objects is tricky as the user program continues to read and write them. Now, this is where Shenandoah is different from other collectors. Evacuation or compacting happens entirely concurrently. This is done to reduce the fragmentation in memory allocation and, hence, is also known as compact. Now, the next step is to move the live objects in the collection set to other regions. The garbage regions are the regions where no live objects are present. Once the marking is complete, the garbage regions are ready to be reclaimed. The final-mark also prepares the collection set that indicates the regions to be evacuated. The parts that happen in the stop-the-world mode are the init-mark to scan the root set and the final-mark to drain all pending queues and re-scan the root set. While most of the marking is done concurrently, there are still some parts that are done in stop-the-world mode. Shenandoah makes use of the SATB barrier to maintain the SATB view of the heap. This means that any object that was alive at the beginning of the marking or that has been allocated since the beginning of marking is considered live. Shenandoah solves this by using the Snapshot At the Beginning (SATB) algorithm. This is because the user program concurrently mutates the object graph while marking is in progress. Marking in the stop-the-world mode is simpler, but it gets complicated in concurrent mode. While traversing, we also assign each object one of three colors: white, grey, or black: We can do this by starting from the root objects and traversing the object graph to find reachable objects. Marking is the process of identifying all objects in the heap or parts of it that are unreachable. It works with copying in the young generation and mark-compact in the old generation: G1 is a generational, parallel, concurrent, and incrementally compacting low-pause collector. Nevertheless, it still has problems leading to unpredictable pauses, requires more CPU time, and is not suitable for a heap larger than 4 GB in size.Īs a long-term replacement for CMS, the Garbage First (G1) collector was introduced in Java 7. It tries to minimize the pause time by doing most of the work concurrently with the user program. It works with copying in the young generation and mark-sweep in the old generation: The Concurrent Mark Sweep (CMS) collector introduced in Java 1.4 is a generational, concurrent, low-pause collector. While providing good throughput, they suffer from the problem of long stop-the-world pauses. They are generational collectors and use copying in the young and mark-compact in the old generation: Some of the traditional garbage collectors include serial and parallel collectors. Nevertheless, the sweet spot really is to find a garbage collector that runs with minimal pauses and provides high throughput - all this with a predictable behavior on heap size that can vary from small to very large! This is a constant struggle that has kept the pace of innovation in the Java garbage collection alive since the early days. Generational collectors then use the stop-the-world collection in the young generation and concurrent collection in the old generation, possibly in increments to reduce pauses. It usually works by dividing the heap space into young and old generations. In fact, most modern-day collectors use a hybrid strategy, where they apply both stop-the-world and concurrent approaches. The latter aims for better latency but compromises on throughput. The former achieves higher throughput at the cost of high latency due to long pauses, also known as stop-the-world pauses. Depending upon the algorithm we use for garbage collection, it can either run while the user program is suspended or run concurrently with the user program.
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