CMS: intermittent timeout running nsk/regression/b4796926

nsk/regression/b4796926 timeouts on linux-amd64 with
-server -Xcomp -XX:+UseCompressedOops -XX:+UseConcMarkSweepGC
during
HS14-b09 / JDK7-b42: Promotion: 64bit (product)
/net/vmsqe.russia/export/jdk/re/7/promoted/ea/b42/binaries/linux-amd64

I narrowed this to
JAVA_OPTS="-XX:+UseConcMarkSweepGC -Xcomp"
both options are necessary.
(this is 64bit Server instance only, so "-d64 -server" are not necessary)

Not reproduced with /net/vmsqe.russia/export/jdk/re/7/promoted/ea/b41/binaries/linux-amd64


The test takes just a few seconds to run if everything is ok,
so it may be considered as timeout after, say, 30 seconds.
jstack -f produces a lot of heap walking exceptions,
it also mentions 4 BLOCKED threads and no deadlocks,
I'm not sure whether this may be trusted.
Only kill -9 is able to kill the java process.

The test just creates and interns 2000000 of strings to exhaust the PermGen.
It is very simple and single threaded, just checks that there's no hangs or crashes. I tried running it with a smaller 4MB PermGen (probably, default is 64 fot this configuration), it hangs too just a bit faster.

see comments on how to reproduce this.