Make work packet buffer size configurable from one location (#1285)

Closes #1281

Author: k-sareen

src/plan/tracing.rs

@@ -1,7 +1,7 @@
 //! This module contains code useful for tracing,
 //! i.e. visiting the reachable objects by traversing all or part of an object graph.
 
-use crate::scheduler::gc_work::{ProcessEdgesWork, SlotOf};
+use crate::scheduler::gc_work::{ProcessEdgesWork, SlotOf, EDGES_WORK_BUFFER_SIZE};
 use crate::scheduler::{GCWorker, WorkBucketStage};
 use crate::util::ObjectReference;
 use crate::vm::SlotVisitor;
@@ -25,7 +25,7 @@ pub struct VectorQueue<T> {
 
 impl<T> VectorQueue<T> {
     /// Reserve a capacity of this on first enqueue to avoid frequent resizing.
-    const CAPACITY: usize = 4096;
+    const CAPACITY: usize = EDGES_WORK_BUFFER_SIZE;
 
     /// Create an empty `VectorObjectQueue`.
     pub fn new() -> Self {

src/scheduler/gc_work.rs

@@ -10,6 +10,14 @@ use crate::*;
 use std::marker::PhantomData;
 use std::ops::{Deref, DerefMut};
 
+/// Buffer size for [`ProcessEdgesWork`] work packets. This constant is exposed to binding
+/// developers so that they can use this value for places in their binding that interface with the
+/// work packet system, specifically the transitive closure via `ProcessEdgesWork` work packets
+/// such as roots gathering code or weak reference processing. In order to have better load
+/// balancing, it is recommended that binding developers use this constant to split work up into
+/// different work packets.
+pub const EDGES_WORK_BUFFER_SIZE: usize = 4096;
+
 pub struct ScheduleCollection;
 
 impl<VM: VMBinding> GCWork<VM> for ScheduleCollection {
@@ -556,7 +564,7 @@ pub trait ProcessEdgesWork:
     /// Higher capacity means the packet will take longer to finish, and may lead to
     /// bad load balancing. On the other hand, lower capacity would lead to higher cost
     /// on scheduling many small work packets. It is important to find a proper capacity.
-    const CAPACITY: usize = 4096;
+    const CAPACITY: usize = EDGES_WORK_BUFFER_SIZE;
     /// Do we update object reference? This has to be true for a moving GC.
     const OVERWRITE_REFERENCE: bool = true;
     /// If true, we do object scanning in this work packet with the same worker without scheduling overhead.