GC_Locker turning CMS into a stop-the-world collector

[Y. Srinivas Ramakrishna]

Just a very quick update on this. Chi Ho and i have been
communicating off-line on this yesterday and today,
and i filed:-

    6919638 CMS: NIO-related performance issue, gc locker implementation suspected

and have updated it with what we know so far (the updates
appear somewhat delayed at bugs.sun.com).

what is known so far is that the problem is mitigated in
jdk 7 because the array copies are broken down into
smaller critical sections; as a result the test case
does not exhibit the blocking behaviour on jdk7.
Not so in jdk 6uXX where the array copies occur in
a single critical section, and the blockage is easily
reproducible.

What is not yet clear to me (although i am looking
into it) is why the blockages seem to coincide with CMS
background collections. Investigation ongoing, and
the bug report will be kept updated as we find out more.

Chi Ho's latest test case (attached to the bug report) was
crucial in quickly reproducing the reported problem (thanks, Chi Ho!).
-- ramki

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