On Sat, 20 Jan 2024 19:48:07 GMT, Volker Simonis <simo...@openjdk.org> wrote:
> Currently we don't record dependencies on redefined methods (i.e. > `evol_method` dependencies) in JIT compiled methods if none of the > `can_redefine_classes`, `can_retransform_classes` or > `can_generate_breakpoint_events` JVMTI capabalities is set. This means that > if a JVMTI agent which requests one of these capabilities is dynamically > attached, all the methods which have been JIT compiled until that point, will > be marked for deoptimization and flushed from the code cache. For large, > warmed-up applications this mean deoptimization and instant recompilation of > thousands if not then-thousands of methods, which can lead to dramatic > performance/latency drop-downs for several minutes. > > One could argue that dynamic agent attach is now deprecated anyway (see [JEP > 451: Prepare to Disallow the Dynamic Loading of > Agents](https://openjdk.org/jeps/451)) and this problem could be solved by > making the recording of `evol_method` dependencies dependent on the new > `-XX:+EnableDynamicAgentLoading` flag isntead of the concrete JVMTI > capabilities (because the presence of the flag indicates that an agent will > be loaded eventually). > > But there a single, however important exception to this rule and that's JFR. > JFR is advertised as low overhead profiler which can be enabled in production > at any time. However, when JFR is started dynamically (e.g. through JCMD or > JMX) it will silently load a HotSpot internl JVMTI agent which requests the > `can_retransform_classes` and retransforms some classes. This will inevitably > trigger the deoptimization of all compiled methods as described above. > > I'd therefor like to propose to *always* and unconditionally record > `evol_method` dependencies in JIT compiled code by exporting the relevant > properties right at startup in `init_globals()`: > ```c++ > jint init_globals() { > management_init(); > JvmtiExport::initialize_oop_storage(); > +#if INCLUDE_JVMTI > + JvmtiExport::set_can_hotswap_or_post_breakpoint(true); > + JvmtiExport::set_all_dependencies_are_recorded(true); > +#endif > > > My measurements indicate that the overhead of doing so is minimal (around 1% > increase of nmethod size) and justifies the benefit. E.g. a Spring Petclinic > application started with `-Xbatch -XX:+UnlockDiagnosticVMOptions > -XX:+LogCompilation` compiles about ~11500 methods (~9000 with C1 and ~2500 > with C2) resulting in an aggregated nmethod size of around ~40bm. > Additionally recording `evol_method` dependencies only increases this size be > about 400kb. The ration remains about the same i... Thanks everybody for looking at this. I've now guarded the feature with a diagnostic command, updated the source code comments around `VM_RedefineClasses::flush_dependent_code` and added an assertion for the new flag. ------------- PR Comment: https://git.openjdk.org/jdk/pull/17509#issuecomment-1904842722
