MPS Wiki: AMC Poolclass

This wiki article contains incomplete and informal notes about the MPS, the precursor to more formal documentation. Not confidential. Readership: MPS users and developers.

Introduction

[Incomplete... Should eventually go into new Guide section of design/poolamc. RHSK 2007-01-18]

See also segment state diagram.

Lots of complex MPS concepts come together in poolamc. This document first states the problem, and then carves it up to arrive a the solution.

The Problem

Input:

• Segment mode:
  • Mobile
  • Board
  • Stuck (segment nailed, but no nailboard)
• Tracing at:
  • Normal
  • Emergency
• Does this fix affect the segment we are scanning?
  • Yes: see wiki:trace/.scan.change
  • No
• Object type:
  • Client
  • Fwd
  • Pad
• Segment rankset:
  • Exact
  • Zero (no refs)
  • Weak
• Object colour:
  • Black for any of N traces
  • Grey for any of N traces
  • White for any of N traces
• Buffers:
  • Buffer types:
    • Mutator Grey
    • Mutator Black
    • Forwarding Grey
  • Segment regions:
    • Unbuffered
    • Trail
    • Raw (from Init to Alloc)
    • Spare (beyond Alloc)

Output

• How to do these Operations:
  • Whiten
  • Scan
  • Fix
  • Reclaim
  • Walk
  • Write-barrier hit on Black
• When to do these segment mode transitions:
  • Mobile -> Board
  • Mobile -> Stuck
  • Board -> Mobile
  • Stuck -> Mobile

Colour

The things that contribute to colour are: SegGrey, SegWhite, SegNailed, segment mode, and (if Board) "nails" (marks in the board).

Buffer colour is an extra complication.

Segment-mode transitions

Mobile mode requires the whole seg to be the same colour. Can the Mobile mode support all the various colour transitions?

Colour transitions

• Atomic whole segment:
  • B→W (whiten)
  • B→G (greyen to make foundation)
• Progressive:
  • G→B (blacken, ie. scan): Attempts whole seg.
• Random:
  • W→G (greyen by fix)
  • W→B (blacken by fix when rank=∅, ie. leaf only)
• Unused:
  • G→W

Board and Stuck modes are used to record ........ er....

.amc.scan.loop:

AMC is expected to mostly avoid this.

Non-boarded segs are always all one colour: AMCScan on these always scans all objects.

Boarded segs:

• in non-emergency mode: we can't be scanning at Ambig (because only roots can be ambig), so if there's a backwards ref that newly finds an object, it will be preserved and greyed (during the scan) by copy into another segment (or at least -- given that buffers are currently attached at the ends of segments only -- into a definitely later part of the same seg).

But in emergency mode, all fixing (even of exact refs) is in-place. In emergency mode, a seg with a board will keep its board. So AMCScan on boarded segments (the misnamed amsScanNailed and amcScanNailedOnce) must loop: do { newMarks = 0; ... } while (newMarks). (Communication between these two happens through the poorly named "moreReturn").

In emergency mode on a mobile (non-boarded) or already stuck seg, .... [what happens here?]

AMS also has a loop: see do { } while (amsseg->marksChanged) in AMSScan.

B. Document History

  2007-01-15  RHSK  Created.
  2007-04-30  RHSK  Link to seg-state diagram.

C. Copyright and License

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