JVM parallel object creation performance

Question

It is said that Java allows to run multiple threads in parallel. It also says that object creation is cheap so that I must always prefer creating new object to reusing them. But, to my knowledge, the objects are created in the global scope (to become a subject to GC). Now, comes the wonder, is parallelism stopped when any of the threads creates an object?

AFAIK, unmanaged languages create the objects on the thread stack so that threads keep running independently. They are all removed once you exit from the subprogram scope. That is you need not add the objects into global list and stop the machine to GC them later. You could do the same with Int/String-like immutable objects in Java, because thy cannot refer other objects creating circular dependencies, that need GC to cleanup. But, afaik, nothing is cleaned up at procedure exit in Java.

The question is still not clear to me, Can you explain me a bit more about what you are asking basically ? — pbajpai
– pbajpai, Commented Jun 16, 2016 at 8:07

Sergey Tselovalnikov · Accepted Answer · 2016-06-18 12:54:22Z

2

Allocation of small objects is quite cheap most of the time because of TLAB (Thread Local Allocation Buffer). Every single thread has a special area in Eden reserved for thread-local allocations called TLAB.

So, you need synchronization only for allocation a new TLAB when previous is filled. That synchronization is a CAS operation, so it is quite fast.

        Eden                       Survivor 1/2
-------------------------------------------------
| T  |     | T  |                ||      |      |
| L  |     | L  |                ||      |      |
| A  |     | A  |                ||      |      |
| B  |     | B  |                ||      |      |
|    |     |    |                ||      |      |
| 1  |     | 2  |                ||      |      |
-------------------------------------------------
  ^           ^
  |           |
  reserved for|thread-1 allocations
              |
              |
              reserved for thread-2 allocations

Moreover, some optimizations can help you to avoid allocations in compiled code - Escape Analysis and Scalar Replacement. In some scenarios compiler can eliminate allocations by placing all the fields of an object on the stack.

edited Jun 18, 2016 at 12:54

answered Jun 16, 2016 at 8:14

Sergey Tselovalnikov

6,0562 gold badges39 silver badges60 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Holger Over a year ago

It might be worth adding that, as long as a thread doesn’t modify heap variables outside the TLAB, it is impossible for any other thread to have a reference to any of the newly created objects in that TLAB. And the JVM tracks exactly that, to enable local or minor collections, which do not need stopping the entire JVM (to address the second paragraph of the question).

Collectives™ on Stack Overflow

JVM parallel object creation performance

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related