MPS issue job001809

TitleMPS ambiguous internal pointer makes AMC pool retain memory as pads
Assigned userRichard Kistruck
DescriptionMPS ambiguous internal pointer makes AMC pool retain memory as pads

Related jobs:
  - job001811 "MPS AMC small nailed objects risk big retention by placement after big objects"

RHSK 2008-04-30
When allocating very large objects (order 10MB), AMC pool can retain
large amounts of memory as pads, apparently because of an ambiguous
internal pointer (that is: a pointer in an ambiguous root, such as
the stack, which points into the middle of an object). Such a
pointershould not and does not preserve the object, but it causes
MPS AMC pool to convert the memory into a pad object, where in some
circumstances the memory could be freed instead.
AnalysisRHSK 2008-04-30
amcReclaimNailed() converts dead objects to pads, but does not call
SegFree(). This is correct and required if some objects are still
alive, or the seg is buffered. But if all objects in the seg are
dead, and the seg is unbuffered, then it should be possible to call

If the seg is small, the retained pad is small. But if the seg
contains a very large object, the amount of retained memory is also
very large.

Fix (in this job): call SegFree() when possible.

Experimental release exp-164852 shows the benefits of calling SegFree.

(A further improvement would be to not put large and small objects
together in a seg. See job001811.)
How foundcustomer
EvidenceSee [pads on heap] <>
and the discussion that followed.
Observed in1.108.1
Introduced in1.100.0
Created byRichard Kistruck
Created on2008-04-30 16:25:58
Last modified byRichard Kistruck
Last modified on2008-05-01 17:12:32
History2008-04-30 RHSK Created.
2008-05-01 RHSK Cross-ref to job001811.


Change Effect Date User Description
164885 closed 2008-04-30 18:56:56 Richard Kistruck MPS master (integ from br/auto_header): make amcReclaimNailed() call
SegFree if the seg had no preserved objects, no buffer, and is no
longer nailed for any trace.
164884 closed 2008-04-30 16:50:12 Richard Kistruck MPS br/auto_header: (cosmetic) tidy up, add AVER unbuffered.
164851 closed 2008-04-29 15:09:12 Richard Kistruck MPS br auto_header: cannot SegFree nailed seg if it has a buffer
164849 open 2008-04-29 14:52:35 Richard Kistruck MPS br auto_header (not the right place for this): fix .nailboard.limitations.middle;
see mail from Configura.