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.