After JDK-8056240 JDK 8u40 may crash during G1's concurrent mark phase if ParallelGCThreads=0:

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/opt/jprt/T/P1/172752.amurillo/s/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp:5231), pid=19641, tid=3810052976
# assert(!MetadataOnStackMark::has_buffer_for_thread(Thread::current())) failed: Should be empty
#
# JRE version: Java(TM) SE Runtime Environment (8.0_40) (build 1.8.0_40-internal-fastdebug-201411071727.amurillo.hs25-40-b18-snapsh-b00)
# Java VM: Java HotSpot(TM) Client VM (25.40-b18-fastdebug compiled mode linux-x86 )
# Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# If you would like to submit a bug report, please visit:
# http://bugreport.java.com/bugreport/crash.jsp
#

--------------- T H R E A D ---------------

Current thread (0xf66ecc00): VMThread [stack: 0xe310c000,0xe318d000] [id=19647]

Stack: [0xe310c000,0xe318d000], sp=0xe318b740, free space=509k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x9d9f5b] VMError::report_and_die()+0x17b
V [libjvm.so+0x389118] report_vm_error(char const*, int, char const*, char const*)+0x68
V [libjvm.so+0x41e116] G1ParallelCleaningTask::work(unsigned int)+0x46
V [libjvm.so+0x40880a] G1CollectedHeap::parallel_cleaning(BoolObjectClosure*, bool, bool, bool)+0x20a
V [libjvm.so+0x336ae7] ConcurrentMark::weakRefsWorkParallelPart(BoolObjectClosure*, bool)+0x27
V [libjvm.so+0x34172f] ConcurrentMark::weakRefsWork(bool)+0x6ef
V [libjvm.so+0x34190b] ConcurrentMark::checkpointRootsFinal(bool)+0xdb
V [libjvm.so+0x376b44] CMCheckpointRootsFinalClosure::do_void()+0x14
V [libjvm.so+0xa05b03] VM_CGC_Operation::doit()+0xd3
V [libjvm.so+0xa0407a] VM_Operation::evaluate()+0x9a
V [libjvm.so+0xa016f4] VMThread::evaluate_operation(VM_Operation*)+0x124
V [libjvm.so+0xa021ca] VMThread::loop()+0x65a
V [libjvm.so+0xa02441] VMThread::run()+0xc1
V [libjvm.so+0x821279] java_start(Thread*)+0x119
C [libpthread.so.0+0x6a49]

VM_Operation (0xe39ff2c8): CGC_Operation, mode: safepoint, requested by thread 0xf663e400


Note that you can't use ParallelGCThreads=0 & G1GC w/ latest JDK 9 builds.