mem-pool-gc-begin and mem-pool-gc-end probes for different memory pools references to one memory manager and it seems to be a memory manager that initiated GC, for example:

  0 53955 __1cPGCMemoryManagerGgc_end6MbbbbnHGCCauseFCause__v_:mem-pool-gc-end mem-pool-gc-end
Pool non-profiled nmethods managed by G1 Young Generation
memory manager ptr: 0xfffffd7ffeb998f0

              libjvm.so`__1cPGCMemoryManagerGgc_end6MbbbbnHGCCauseFCause__v_+0x1c2
              libjvm.so`__1cPG1CollectedHeapbGdo_collection_pause_at_safepoint6Md_b_+0xc5a
              libjvm.so`__1cXVM_G1IncCollectionPauseEdoit6M_v_+0x194
              libjvm.so`__1cMVM_OperationIevaluate6M_v_+0x122
              libjvm.so`__1cIVMThreadSevaluate_operation6MpnMVM_Operation__v_+0x20b
              libjvm.so`__1cIVMThreadEloop6M_v_+0x7d1
              libjvm.so`__1cIVMThreadDrun6M_v_+0xa9
              libjvm.so`java_start+0x10d
              libc.so.1`_thr_setup+0x5b
              libc.so.1`_lwp_start

According to documentation it should be "the manager which manages this memory pool".
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/dtrace.html

And "G1 Young Generation" is definitely not the memory manager for "non-profiled nmethods" code cache heap.

Moreover, it seems like that these probes are never emitted by the NMethodSweeper.