Andy C. King and Richard Jones

University of Kent at Canterbury

Removing GC Synchronisation

Overview

The Problem

An Observation

L & S Heaplets

Implementation

A Fly in the Ointment

Swatting the Fly

Measurements

Future Work

Concurrency in multi-threaded applications

Synchronisation is necessary

Language level• synchronized

VM level• mutator and garbage collector (GC) threads• suspending threads before collecting

the latter can be quite costly

The Problem

Objects may be thread-local (L)

Reachable only within their allocating thread

do not need synchronisation for such objects

Can manage these objects locally

Place in per-thread heap regions called heaplets

No need to stop other threads to collect such objects

Shared objects (G) go in global heap

An Observation

T1

T2

G L1

L2

L & G Heaplets

Variation on Steensgaard [Stee00]

Escape analysis technique• automatically determine local objects• mark their allocation sites• patch to use appropriate heaplet

Flow-insensitive

Context-sensitive (specialisation)

performed at runtime

Implementation

Implementation (2)

Operates on a snapshot of the world• set of classes loaded at that point in time

JDK 1.22_06 Production Release for Solaris• new GC with heaplets• support specialisation• patches applied in interpreter• patched/specialised methods can be JIT’ed

Dynamic class loading

Class loaded after snapshot

May override method containing L allocation site

Shares the object

L allocation site now S

(transitive closure)

A Fly in the Ointment

1 class A {2 public void do(Object o) { }3 }4 class Main {5 public static void main(String[] args) {6 A a = new A();7 foo(a); SNAPSHOT ANALYSIS30 A a’ = new A’(); LOAD CLASS A’31 foo(a’);32 }33 public static void foo(A inA) {34 Object o = new Object();35 inA.do(o);36 }37 }

A Fly in the Ointment (2)

Truly shared (G)• reachable from >1 thread• G heap

Truly local (L)• reachable from exactly 1 thread• can prove type• L heaplet

Optimistically local (OL)• reachable from 1 thread• cannot prove type• OL heaplet

Swatting the Fly

T1

T2

G L1

L2

OL1

OL2

Swatting the Fly (2)

Analyse new classes immediately

Compare methods

If mismatched, OL sites are broken

Class is non-conforming

OL heaplet becomes G

can be done very cheaply

only OL heaplets of referring threads are affected

Swatting the Fly (3)

T1

T2

G L1

L2

OL1

OL2

Swatting the Fly (4)

Measurements

Classes (resolved) Methods (resolved)% %

Volano client 396 67.17 2946 94.60Volano server 514 67.70 4309 95.57SPECjbb 642 73.36 5365 96.18

Specs Bytecode +Bloat Compiled +BloatKB % KB %

Volano client 680 112 26.79 580 54.66Volano server 3193 196 82.14 657 181.58SPECjbb 2315 286 43.71 1976 68.67

Measurements (2)

L OL GVolano client 51 307 39Volano server 320 1224 474SPECjbb 281 1374 154

Started Elapsed Memorys s MB

Volano client 10 1.5 2.5Volano server 5 2.3 4.2SPECjbb 30 4.4 6.2

Finally get the bloody numbers from the GC…

Future Work

Andy C. King and Richard Jones

University of Kent at Canterbury

Q&A