With Graal on JVMTI clients start seeing JVMCI compiler threads and get notified about events these threads post and breakpoints they hit, that makes the debugging experience quite complicated comparing to the case when Graal is off. It would be useful if by default JVMTI hid/ignored/muted JVMCI compiler threads (and probably some other "system" threads) and provided a way for JVMTI client to "unmute" these threads when required.