www.ravenbrook.com

BAD BEHAVIOUR IN THE MPS

Richard Kistruck, Ravenbrook Limited, 2006-12-10

1. INTRODUCTION

This is a brainstorm about what could cause bad behaviour in the MPS, from RHSK considering the Configura shark-fin problem [shark-fin].

This document is Confidential. Readership: Ravenbrook staff, Configura staff. (But the intention is to produce a declassified version of this document).

CONTENTS

1. Introduction & Contents
2. Classification
3. Observations
4. Brainstorm
A. References
B. Document history
C. Copyright and license

2. CLASSIFICATION

Tuning that affects:

Ta-trigger: triggering of a collection (eager / lazy)

Tb-whiten: condemned-set of a collection

Tc-rate: trace->rate of a collection (eager / lazy)

Td-labour: how laborious the collection is (if remembered sets are poor)

Te-nailed: how much is nailed via ambiguous refs or barrier hits at RankAMBIG

Tf-emerg: emergency tracing, and other causes of conservatively retaining garbage.

Defects that:

Da-slow: make tracing slow (eg. N^2, pointless repeats, unnecessary work)

Db-garbage: preserve garbage.

3. OBSERVATIONS

Working-set size (WSS) of Configura's process grows during a compilation. At various points there is a 'shark-fin': a linear-with-time rise that very roughly doubles WSS, followed a linear-with-time drop at an only slightly faster rate, to about the pre-shark-fin level.

The first process run ("extension_build0.jpg") does a lot of work, and there are three shark-fins. In each case the WSS before and after is the same.

Having run once, later process runs (runs 20, 21, and 22: "extension_build 20-22.jpg") do less work. Near the end of each, plus once in the middle of 21, there is a shark fin. Some shark-fins result in WSS decrease, others in increase. The WSS at end of run grows: after 21 bigger than after 20, and after 22 is bigger still.

Could these shark-fins be full collections?

If so, could bad tuning, or defects, in the MPS cause these full-collections to not collect garbage properly?

4. BRAINSTORM

A. REFERENCES

[shark-fin] "Full collection problems"; rhsk; 2006-12-10; <http://info.ravenbrook.com/mail/2006/12/08/15-57-33/0.txt>

[#REF_NAME#] "#REF_TITLE#"; #REF_AUTHOR#; #REF_DATE#; <#REF_URL#>

B. DOCUMENT HISTORY

2006-12-10 RHSK Created from thinking about [shark-fin].

C. COPYRIGHT AND LICENSE

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