JDK-6755514 : CMS: sharpen asserts related to free list consistency and locking
  • Type: Bug
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: hs10,hs14
  • Priority: P3
  • Status: Closed
  • Resolution: Duplicate
  • OS: generic
  • CPU: generic
  • Submitted: 2008-10-03
  • Updated: 2010-05-11
  • Resolved: 2008-10-29
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JDK 7
7Resolved
Related Reports
Duplicate :  
JDK-6631166 - CMS: better heuristics when combatting fragmentation
Duplicate :  
JDK-6755512 - CMS: unsafe generation expansion
Relates :  
JDK-6755518 - CMS: mysterious memory corruption in core files
Description
Cores from a customer revealed :
(1) a memory corruption issue
(2) a free list inconsistency

There's a separate bug tracking the memory corruption issue.
This bug is filed because it is possible that the free list
inconsistency may be orthogonal and independent of the
memory corruption issue.

The core files of significance are the ones from 9/18 and 9/22.

The free list inconsistency may be described thus:

  . the free lists in question are those in the small size lists
    indexed from a table of sizes
  . each list has a head and tail pointer to the list of free blocks
    and a count that tracks the list population
  . in both cases in question, the lists were empty but the
    counts indicated non-zero counts

Targeted testing with assertions to check for the correspondence of
counts and list lengths are in progress. It is also possible to
make the code "self-healing" in that if an inconsistency is discovered
it can be fixed without crashing, but at the cost of some performance.
See Evaluation section.

The Synopsis field is accordingly being changed to reflect the work in this
CR from:-

CMS: free list inconsistency in core file

to:-

CMS: sharpen asserts related to free list consistency and locking
Comments
EVALUATION Since the code is in the workspace related to 6631166, i'll close this as a duplicate of 6631166, whence the code should find its way into the repository.
29-10-2008
SUGGESTED FIX The sharpened assertions are part of the following workspace:- http://analemma.sfbay.sun.com/net/spot/workspaces/ysr/frag and will be rolled in with the putback for 6631166 (which is the CR for the bulk of the modifications in the above workspace).
23-10-2008
EVALUATION Assertions related to the consistency of free lists and their locking were tightened considerably. However, none of these revealed any issues. The bug will be closed after pushing the tightened assertions into the code base. However, this no longer qualifies as a P2 because in the case of this customer the crashes had nothing to do with the JVM, let alone the CMS garbage collector; see 6755518 and cousins.
23-10-2008