Summary

Change the JVM TI ThreadStart/ThreadEnd events so they are only sent for platform threads. Agents that wish to get events when virtual Threads start or end can enable the VirtualThreadStart/VirtualThreadEnd events.

Problem

It is possible to create much more virtual threads than platform threads as they are light wight. In such cases the execution of ThreadStart/ThreadEnd event callbacks will give a significant performance overhead. To overcome this potential problem the VirtualThreadStart/VirtualThreadEnd events for virtual trheads can be enabled/disabled separately from the ThreadStart/ThreadEnd events for platform threads. The can_support_virtual_threads has to be enabled for that. If this capability is disabled then the ThreadStart/ThreadEnd events will be generated for virtual threads as well as for platform threads. This was for compatibility with old/legacy JVMTI agents which were not upgraded to support virtual threads. The problem with this is that there can still can be a unexpectedly big performance overhead when profiled applications create a lot of virtual threads.

Solution

The solution is to get rid of the compatibility ThreadStart/ThreadEnd events for virtual threads when the can_support_virtual_threads capability is disabled. The existing agents are expected to be upgraded to support virtual threads and enable the can_support_virtual_threads capability if they need to be notified about virtual threads creation and termination.

Specification

There are the following changes in the JVMTI spec:

• ThreadStart event spec:

Replace the fragment:

This event is generated by platform threads. It is also generated by
`virtual threads when the capability `can_support_virtual_threads`
`can_support_virtual_threads` is not enabled. Agents without support
for virtual threads that enable this event will therefore be notified by all
newly started threads. If the capability `can_support_virtual_threads` is
enabled then this event is not generated by virtual threads.
Agents with support for virtual threads can enable `VirtualThreadStart`
to be notified by newly started virtual threads.

with the fragment:

This event is generated by platform thread. It is not generated by virtual threads.
Agents with the `can_support_virtual_threads` capability can enable the
VirtualThreadStart event to be notified by newly started virtual threads.

• ThreadEnd event spec:

Replace the fragment:

This event is generated by platform threads. It is also generated by
`virtual threads when the capability `can_support_virtual_threads`
`can_support_virtual_threads` is not enabled. Agents without support for
virtual threads that enable this event for all threads will therefore be notified
by all terminating threads. If the capability `can_support_virtual_threads` is
enabled then this event is not generated by virtual threads.
Agents with support for virtual threads can enable `VirtualThreadEnd`
to be notified by terminating virtual threads.

with the fragment:

This event is generated by platform thread. It is not generated by virtual threads.
Agents with the `can_support_virtual_threads` capability can enable the
VirtualThreadEnd event to be notified by newly started virtual threads.

The jvmti.xml update is:

diff --git a/src/hotspot/share/prims/jvmti.xml b/src/hotspot/share/prims/jvmti.xml
index 456876fbcb9..546b45ac86e 100644
--- a/src/hotspot/share/prims/jvmti.xml
+++ b/src/hotspot/share/prims/jvmti.xml
@@ -12937,16 +12940,10 @@ myInit() {
       A thread start event is generated by a new thread before its initial
       method executes.
       <p/>
-      This event is generated by platform threads. It is also generated by
-      virtual threads when the capability
-      <internallink id="jvmtiCapabilities.can_support_virtual_threads">
-      <code>can_support_virtual_threads</code></internallink> is not enabled.
-      Agents without support for virtual threads that enable this event will
-      therefore be notified by all newly started threads.
+      This event is generated by platform thread. It is not generated by virtual threads.
       <p/>
-      If the capability <code>can_support_virtual_threads</code> is enabled then
-      this event is not generated by virtual threads. Agents with support for
-      virtual threads can enable <eventlink id="VirtualThreadStart"></eventlink>
+      Agents with the <code>can_support_virtual_threads</code> capability
+      can enable the <eventlink id="VirtualThreadStart"></eventlink> event
       to be notified by newly started virtual threads.
       <p/>
       A platform thread may be listed in the array returned by
@@ -12984,16 +12981,10 @@ myInit() {
       A thread end event is generated by a terminating thread after its
       initial method has finished execution.
       <p/>
-      This event is generated by platform threads. It is also generated by
-      virtual threads when the capability
-      <internallink id="jvmtiCapabilities.can_support_virtual_threads">
-      <code>can_support_virtual_threads</code></internallink> is not enabled.
-      Agents without support for virtual threads that enable this event for
-      all threads will therefore be notified by all terminating threads.
+      This event is generated by platform thread. It is not generated by virtual threads.
       <p/>
-      If the capability <code>can_support_virtual_threads</code> is enabled then
-      this event is not generated by virtual threads. Agents with support for
-      virtual threads can enable <eventlink id="VirtualThreadEnd"></eventlink>
+      Agents with the <code>can_support_virtual_threads</code> capability
+      can enable the <eventlink id="VirtualThreadSEnd"></eventlink> event
       to be notified by terminating virtual threads.
       <p/>
       A platform thread may be listed in the array returned by