Shenandoah GC in JDK 13, Part I: Load Reference Barriers

May 15, 2019

In this miniseries, I’d like to introduce a couple of new developments of the Shenandoah GC that are upcoming in JDK 13. Perhaps the most significant, even though not directly user-visible, change is the switch of Shenandoah’s barrier model to load reference barriers. It resolves one major point of criticism against Shenandoah, that is their expensive primitive read-barriers.

Shenandoah (as well as other collectors) employ barriers in order to ensure heap consistency. More specifically, Shenandoah GC employs barriers to ensure what we call ‘to-space-invariant’. What it means is this: when Shenandoah is collecting, it is copying objects from so-called ‘from-space’ to ‘to-space’, and it does so while Java threads are running (concurrently). This means that there may be two copies of any object floating around in the JVM. In order to maintain heap consistency, we need to ensure either of:

writes happen into to-space copy + reads can happen from both copies, subject to memory model constraints = weak to-space invariant

writes and reads always happen into/from the to-space copy = strong to-space invariant

And the way we ensure that is by employing the corresponding type of barriers whenever reads and writes happen. Consider this pseudocode:

void example(Foo foo) { Bar b1 = foo.bar; // Read while (..) { Baz baz = b1.baz; // Read b1.x = makeSomeValue(baz); // Write }

Employing the Shenandoah barriers, it would look like this (what the JVM+GC would do under the hood):

void example(Foo foo) { Bar b1 = readBarrier(foo).bar; // Read while (..) { Baz baz = readBarrier(b1).baz; // Read X value = makeSomeValue(baz); writeBarrier(b1).x = readBarrier(value); // Write }

I.e. whereever we read from an object, we first resolve the object via a read-barrier, and wherever we write to an object, we possibly copy the object to to-space. I won’t go into the details of this here, let’s just say that both operations are somewhat costly. Notice also that we need a read-barrier on the value of the write here to ensure we only ever write to-space-references into fields while heap references get updated (another nuisance of Shenandoah’s old barrier model).

Seeing that those barriers are a costly affair, we worked quite hard to optimize them. A very important optimization is to hoist barriers out of loops. We see that b1 is defined outside the loop, but only used inside the loop. We can just as well do the barriers outside the loop, once, instead of many times inside the loop:

void example(Foo foo) { Bar b1 = readBarrier(foo).bar; // Read Bar b1' = readBarrier(b1); Bar b1'' = writeBarrier(b1); while (..) { Baz baz = b1'.baz; // Read X value = makeSomeValue(baz); b1''.x = readBarrier(value); // Write }

And because write-barriers are stronger than read-barriers, we can fold the two up:

void example(Foo foo) { Bar b1 = readBarrier(foo).bar; // Read Bar b1' = writeBarrier(b1); while (..) { Baz baz = b1'.baz; // Read X value = makeSomeValue(baz); b1'.x = readBarrier(value); // Write }

This is all nice and works fairly well, but it is also troublesome: the optimization passes for this are very complex. The fact that both from-space and two-space-copies of any objects can float around the JVM at any time is a major source of headaches and complexity. For example, we need extra barriers for comparing objects in case we compare an object to a different copy of itself. Read-barriers and write-barriers need to be inserted for *any* read or write, including primitive reads or writes. And those are very frequent, especially reads.

So why not short-cut this, and strongly ensure to-space-invariance right when an object is loaded from memory? That is where load-reference-barriers come in. They work mostly like our previous write-barriers, but are not employed at use-sites (when reading from or storing to the object), but instead much earlier when objects are loaded (at their definition-site):

void example(Foo foo) { Bar b1' = loadReferenceBarrier(foo.bar); while (..) { Baz baz = loadReferenceBarrier(b1'.baz); // Read X value = makeSomeValue(baz); b1'.x = value; // Write }

You can see that the code is basically the same as before – after our optimizations- , except that we didn’t need to optimize anything yet. Also, the read-barrier for the store-value is gone, because we now know (because of the strong to-space-invariant) that whatever makeSomeValue() did, it must already have employed the load-reference-barrier if needed. The new load-reference-barrier is almost 100% the same as our previous write-barrier.

The advantages of this barrier model are many (for us GC developers):

Strong invariant means it’s a lot easier to reason about the state of GC and objects

Much simpler barrier interface. Infact, a lot of stuff that we added to GC barrier interfaces after JDK11 will now become unused: no need for barriers on primitives, no need for object equality barriers, etc.

Optimization is much easier (see above). Barriers are naturally placed at the least-hot locations: their def-sites, instead of their most-hot locations: their use-sites, and then attempted to optimize them away from there (and not always successfully).

No more need for object equals barriers

No more need for ‘resolve’ barriers (a somewhat exotic kind of barriers used mostly in intrinsics and places that do read-like or write-like operations)

All barriers are now conditional, which opens up opportunities for further optimization later on

We can re-enable a bunch of optimizations like fast JNI getters that needed to be disabled before because they did not play well with possible from-space references

For users, this is mostly invisible, and the bottom line is that this improves overall Shenandoah’s performance. It also opens the way for follow-up improvements like elimination of the forwarding pointer, which I’ll get to in a follow-up article.

Load reference barriers have been integrated into JDK 13 development repository in April 2019. We will start backporting it to Shenandoah’s JDK 11 and JDK 8 backports soon. If you don’t want to wait, you can already have it: check out The Shenandoah GC Wiki.