Concurrency of heap memory allocation

In the actual development process , Often create objects , As a virtual machine , Thread safety must be guaranteed . Generally speaking, virtual machines use two ways to ensure thread safety

One 、CAS + Failure to retry

CAS It is an implementation of optimistic lock , Each time you do not lock, it is assumed that there is no conflict to complete an operation , If the operation fails due to a conflict, try again , Until we succeed . See blog , Delivery address ：Java Concurrent programming unlocked （CAS）

Two 、TLAB

TLAB Is in Java The garden of Eden in the heap space is divided for each Threads Of memory space , Specifically allocate memory in this area for objects created by this thread .

Its main purpose is to allocate memory in a concurrent environment , Reduce competition for memory space between threads , Speed up memory allocation .

TLAB In essence, it's still Java Heap , So in TLAB The object of the area , It can also be accessed by other threads .

If not used TLAB, Multiple threads executing concurrently create objects , When allocating memory , Are likely to Java Apply at the same location in the heap , This requires locking this part of the memory space or adopting CAS And other operations to ensure thread safety , That is, ensure that the region is allocated to only one thread .

Used TLAB after ,JVM In heap memory for each thread reserve A memory area , When reservation occurs , Need to lock or use CAS And so on , Avoid multiple threads reserving the same area . Once a region is assigned to a thread , After that, when the thread needs to allocate memory , Will give priority to applying in this area . This area is for this thread Allocate memory This operation is thread private , Therefore, locking and other operations are not required during allocation , This not only protects thread safety, but also improves the allocation speed .

Be careful ： When the object created by this thread is greater than TLAB Memory remaining in or TLAB When I ran out of memory , Then use CAS + Failed to allocate memory by retry .