GC Points in a Threaded Environment

Many garbage-collected systems, including most that involve a stop-the-world phase, restrict GC to socalled GC points. In single-threaded environments, GC points carry no overhead: when a GC must be done, the single thread is already at a GC point. In multi-threaded environments, however, only the thread that triggers the GC by failing an allocation will be at a GC point. Other threads must be rolled forward to their next GC point before the GC can take place. We compare, in the context of a high-performance JavaTM virtual machine, two approaches to advancing threads to a GC point, polling and code patching, while keeping all other factors constant. Code patching outperforms polling by an average of 4.7% and sometimes by as much as 11.2%, while costing only slightly more compiled code space. Put differently, since most programs spend less than 1/5 of the time in GC, a 4.7% bottom-line speedup amounts to more than a 20% reduction in the GC-related costs. Patching is, however, more difficult to implement. email address: [email protected] © 1998 Sun Microsystems, Inc. All rights reserved. The SML Technical Report Series is published by Sun Microsystems Laboratories, of Sun Microsystems, Inc. Printed in U.S.A. Unlimited copying without fee is permitted provided that the copies are not made nor distributed for direct commercial advantage, and credit to the source is given. Otherwise, no part of this work covered by copyright hereon may be reproduced in any form or by any means graphic, electronic, or mechanical, including photocopying, recording, taping, or storage in an information retrieval system, without the prior written permission of the copyright owner. TRADEMARKS Sun, Sun Microsystems, the Sun logo, and Java are trademarks or registered trademarks of Sun Microsystems, Inc. in the U.S. and other countries. All SPARC trademarks are used under license and are trademarks or registered trademarks of SPARC International, Inc. in the U.S. and other countries. Products bearing SPARC trademarks are based upon an architecture developed by Sun Microsystems, Inc. For information regarding the SML Technical Report Series, contact Jeanie Treichel, Editor-in-Chief .