RFR: 8328866: Add raw monitor rank support to the debug agent. [v4]

[Serguei Spitsyn]

On Wed, 8 May 2024 18:27:15 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> src/jdk.jdwp.agent/share/native/libjdwp/debugLoop.c line 66:
>> 
>>> 64: debugLoop_initialize(void)
>>> 65: {
>>> 66:     vmDeathLock = debugMonitorCreate(vmDeathLockForDebugLoop_Rank, "JDWP VM_DEATH Lock");
>> 
>> Nit: Just a suggestion to consider (there was a similar suggestion from by Alex)...
>> Can all the `debugMonitor's` created/initialized by one function `debugMonitor_initialize()` called by util_initialize()?  Then each monitor consumer can cache needed `debugMonitor's` like this:
>> 
>> vmDeathLock = getDebugMonitor(vmDeathLockForDebugLoop_Rank);
>
> I think you are suggesting that all monitors be created up front during util_intialize(). The debug agent in the past has always created them lazily (and sometimes some of them end up never being created because they are not needed). I don't really see a big advantage in creating them all up front.
> 
> Alex's suggestion was a very different one, and was just a suggestion to initialize all the DebugRawMonitor ranks up front rather than when the jMonitorID is created, and the reason for the suggestion was to avoid having to grab the dbgRawMonitor when setting the rank, but I wasn't convinced that it actually allowed you to not grab the dbgRawMonitor.

I just wanted to say that Alex's suggestion is in a similar direction.
I do not see a big advantage to save just on creation of a few monitors. It is not a scalability problem as their number is constant. Creation of the monitors at initialization time is a simplification as we do not care about synchronization at this phase. Also, it is easier to understand because there is no interaction with other threads.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19044#discussion_r1594571559