THE DESIGN OF THE GENERIC SCANNER
                            design.mps.scan
                           incomplete design
                           richard 1995-08-25

SUMMARIES

Scanned Summary

.summary.subset: The summary of reference seens by scan (ss.unfixedSummary) is 
a subset of the summary previously computed (SegSummary).

There are two reasons that it is not an equality relation:  

1. If the segment has had objects forwarded onto it then its summary will get 
unioned with the summary of the segment that the object was forwarded from.  
This may increase the summary.  The forwarded object of course may have a 
smaller summary (if such a thing were to be computed) and so subsequent 
scanning of the segment may reduce the summary.  (The forwarding process may 
erroneously introduce zones into the destination's summary).  

2. A write barrier hit will set the summary to RefSetUNIV.  

The reason that ss.unfixedSummary is always a subset of the previous summary is 
due to an "optimization" which has not been made in TraceFix.  See 
impl.c.trace.fix.fixed.all.


Partial Scans

.clever-summary: With enough cleverness, it's possible to have partial scans of 
condemned segments contribute to the segment summary.  [We had a system which 
nearly worked -- see MMsrc(MMdevel_poolams at 1997/08/14 13:02:55 BST), but it 
did not handle the situation in which a segment was not under the write barrier 
when it was condemned.]

.clever-summary.acc: Each time we partially scan a segment, we accumulate the 
post-scan summary of the scanned objects into a field in the group, called 
'summarySoFar'.  The post-scan summary is (summary \ white) U fixed.

.clever-summary.acc.condemn: The cumulative summary is only meaningful while 
the segment is condemned.  Otherwise it is set to RefSetEMPTY (a value which we 
can check).

.clever-summary.acc.reclaim: Then when we reclaim the segment, we set the 
segment summary to the cumulative summary, as it is a post-scan summary of all 
the scanned objects.

.clever-summary.acc.other-trace: If the segment is scanned by another trace 
while it is condemned, the cumulative summary must be set to the post-scan 
summary of this scan (otherwise it becomes out-of-date).

.clever-summary.scan: The scan summary is expected to be a summary of all 
scanned references in the segment.  We don't know this accurately until we've 
scanned everything in the segment.  So we add in the segment summary each time.

.clever-summary.scan.fix: TraceScan also expects the scan state fixed summary 
to include the post-scan summary of all references which were white.  Since we 
don't scan all white references, we need to add in an approximation to the 
summary of all white references which we didn't scan.  This is the intersection 
of the segment summary and the white summary.

.clever-summary.wb: If the cumulative summary is smaller than the mutator's 
summary, a write-barrier is needed to prevent the mutator from invalidating it. 
 This means that sometimes we'd have to put the segment under the write-barrier 
at condemn [this is not an operation currently available to pool class 
implementations pekka 1998-02-26], which might not be very efficient.

.clever-summary.method.wb: We need a new pool class method, called when the 
write barrier is hit (or possibly any barrier hit).  The generic method will do 
the usual TraceAccess work, the trivial method will do nothing.

.clever-summary.acc.wb: When the write barrier is hit, we need to correct the 
cumulative summary to the mutator summary.  This is approximated by setting the 
summary to RefSetUNIV.