CMS Concurrent mode failure fallback to the serial old collector?

[Haim Yadid]

I am analysing a GC pause problem and I have noticed that when CMS is used
and a concurrent mode failure occurs or GC is triggered manually (by
System.gc()) the STW collector used does not seem to be parallel. ( I am
aware of the ExplicitGCInvokesConcurrent flag but it will not solve
concurrent failure ).
I tried to play with -XX:ParallelGCThreads=... -XX:ParallelCMSThreads=...
but they seem have no effect (only on the ParNew GC).

I am deducing it from the following GC log line

24.904: [Full GC (System) 24.904: [CMS: 302703K->303056K(2116864K),
1.0847520 secs] 484492K->303056K(2423552K), [CMS Perm :
7528K->7525K(21248K)], 1.0852780 secs] [Times: user=1.04 sys=0.02,
real=1.09 secs]
If it would have been parallel "user" would have been equal to "nThreads" *
"real".
In addition if I choose ParallelOld GC it will behave correctly.

I really do not understand why the failover STW mechanism of CMS is not
parallel shouldn't it be finishing the work as soon as possible ?
I am not able to find anything useful on the internet.

I think G1 behaves in the same manner BTW ( AFAIK the the fallback
collector of G1 is copied from CMS)

Help will be appreciated.
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