Request for review (S): 7029036: Card-table verification hangs with all framework collectors, except G1, even before the first GC

Y. Srinivas Ramakrishna y.s.ramakrishna at oracle.com
Thu Mar 24 01:15:10 PDT 2011


I'd like a few reviews for a fix to the following CR:-

   7029036: Card-table verification hangs with all framework collectors, except G1, even before the 
first GC

Problem: This is a day one performance bug in card-table verification infrastructure
in the so-called "gc framework", and affects CMS, ParNew and SerialGC.
Details on why the problem was unmasked only recently can be found in
the bug report. In a nutshell, existing code has quadratic complexity
when scanning large arrays because for an object of size O(n)
and with k clean card ranges, the cost is O(k.n), which with O(n)
worst case clean card ranges would yield O(n^2) cost.

Solution: By using a memory-interval-bounded iteration over the
clean card ranges of these objects, the complexity is returned
to O(k) which is O(n), because we do not end up scanning an
entire prefix of the object each time before scanning each
clean range.

I also simplified the verification closure (for the product build)
by ensuring that the iterator never returns reference locations
outside the range of clean cards to the closure, which itself just
asserts in non-product builds that this is always the case.
(This assertion allowed us to find a few memory-interval-delimited
iteration methods which were not respecting the memory-interval provided.
That is being separately fixed in CR 7034035; stay tuned.)

Testing: The tests listed in the bug report with +VerifyBefore/AfterGC
to verify that the heap verification now does not "hang" but rather
completes very quickly even with much larger heaps. Also refworkload
on solaris/x86 and JPRT testing (ongoing) with
heap verification switched on.

webrev: http://cr.openjdk.java.net/~ysr/7029036/webrev.00/

Thanks for your reviews!
-- ramki


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