EA feedback

[John Rose]

Here’s the way I would prefer to think about a “dump command”.

The native way that the JVM represents sequential operations is
the method.  Talking about methods is therefore a basic way to
specify a condition for injecting a JVM operation like training
dumps.  I would like to figure out a good way to tie the training
dump to the invocation of a method, either a single well-known
method, or to a method specified (on the command line) by the
user.

In fact, it feels like a breakpoint-like operation would be a
natural way to view the training dump.  You don’t need JVMTI
to get it done; you just need a hack in the VM which parallels
the existing breakpoint mechanism, but special-cases it to
drive a training dump.

Given such a foundation, jsig could then inject a call to a
method which is appropriately tied to the dump command.

Sketch of implementation:

When a method is first linked, a list is checked to see if
it has a dump event tied to it, and a bit is set on the method.
The method’s interpreter entry point might be modified, or
perhaps the interpreter just always checks the bit.  On entry
to the method, before the first bytecode, an upcall tells
the VM that it’s time to finish the training run.

The compilers also check this bit, of course.

There is some method deep in the privates of java.base
that is always treated this way.  That’s what jcmd reaches.
There is a command line option which lists more methods
to treat this way, something like the CompileOnly command.

As a separate option, the upcall to end the training run
might return (allowing the VM to continue) or just exit.

As a separate option, allow the user to specify a count N,
so that the training dump happens only after N “hits” on
any marked method(s).

I think all this is useful and flexible.

On 13 Aug 2024, at 18:22, ioi.lam at oracle.com wrote:

> On 8/13/24 12:42 PM, Ashutosh Mehra wrote:
>>
>>     Being able to trigger assembly/verification via jcmd without
>>     exiting, would make this far easier for us to support.
>>
>> There is a proposed enhancement for doing exactly this (and exploring 
>> other ways to trigger end of training run); see 
>> https://bugs.openjdk.org/browse/JDK-8335358
>
>
> I am working on a prototype for dumping with jcmd. It will be similar 
> to the existing "jcmd VM.cds statoc_dump" command, except that it will 
> also support the dumping of the AOT cache and profile data.
>
>
> Thanks
>
> - Ioi
>
>
>>
>> Thanks,
>> - Ashutosh Mehra
>>
>>
>> On Fri, Aug 9, 2024 at 4:38 PM Danny Thomas <dannyt at netflix.com> 
>> wrote:
>>
>>     I tried 24-leydenpremain+2-8 on a few internal applications, 
>> some
>>     quick feedback below (good to see you folks at the JVM LS!).
>>
>>     If a jar has a Class-Path attribute and one or more of those
>>     libraries are explicitly on the classpath, it causes the actual
>>     and expected classpath to always differ. This is also the case
>>     currently with CDS of course, but this feature is sure to be
>>     deployed far more broadly than CDS is currently, so likely
>>     something you want to look at:
>>
>>     [0.057s][info][class,path] non-existent Class-Path entry
>>     lib/failureaccess-1.0.1.jar
>>     [0.057s][info][class,path] opened:
>>     lib/listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar
>>     [0.057s][info][class,path] library =
>>     lib/listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar
>>
>>     Startup time when training seems to be on par
>>     with ArchiveClassesAtExit in JDK 21, but it's about a 3.5x 
>> startup
>>     time penalty for one of our typical Spring Boot applications. 
>> From
>>     a back-to-back run on my machine (AMD EPYC 9R14, 32 cores, 123G,
>>     Ubuntu 22.04.4 LTS):
>>
>>     Started App in 7.698 seconds (process running for 8.229)
>>     Started App in 26.247 seconds (process running for 29.262) - w/
>>     CacheDataStore, Training Run
>>     Started App in 4.341 seconds (process running for 4.917)  - w/
>>     CacheDataStore, Production Run
>>
>>     I also got a crash on one attempt, I can't remember what I did to
>>     cause this unfortunately:
>>
>>     Stack: [0x00007f3949ab0000,0x00007f3949bb0000],
>>      sp=0x00007f3949bae628,  free space=1017k
>>     Native frames: (J=compiled Java code, j=interpreted, Vv=VM code,
>>     C=native code)
>>     V  [libjvm.so+0x42ca30]
>>      ArchiveBuilder::get_buffered_addr(unsigned char*) const+0x40
>>     V  [libjvm.so+0xce4aa5] 
>>  VM_PopulateDumpSharedSpace::doit()+0x395
>>     V  [libjvm.so+0x100ae69]  VM_Operation::evaluate()+0x109
>>     V  [libjvm.so+0x100e348]
>>      VMThread::evaluate_operation(VM_Operation*)+0xe8
>>     V  [libjvm.so+0x10142fb] 
>>  VMThread::inner_execute(VM_Operation*)+0x35b
>>     V  [libjvm.so+0x101460f]  VMThread::run()+0x16f
>>     V  [libjvm.so+0xf6e5cf]  Thread::call_run()+0x9f
>>     V  [libjvm.so+0xd74e13]  thread_native_entry(Thread*)+0x183
>>     C  [libc.so.6+0x98b07]
>>
>>     siginfo: si_signo: 11 (SIGSEGV), si_code: 1 (SEGV_MAPERR),
>>     si_addr: 0x0000000000000030
>>
>>     Thinking ahead to operationalizing AOT, while a
>>     single-shot/on-exit workflow is great for iterating locally,
>>     requiring the VM to exit makes this more difficult to
>>     operationalize at scale:
>>
>>      1. We'll perform training and assembly on test, production 
>> canary
>>         and production instances on behalf of application owners and
>>         handle distribution of the archives. Depending on when we're
>>         able to perform a training run, it'll have different 
>> benefits.
>>         i.e.:
>>          1. Test environment will at least improve startup
>>             performance, with a mixed benefit for warm up depending 
>> on
>>             the kind of traffic they take in test
>>          2. If an application uses canary deployments we'll have a
>>             full production profile prior to the full production
>>             deployment, and all instances will come up hot
>>          3. If we reach production with only a test environment
>>             profile, we'll perform a training run in production, so
>>             instances that scale up following that run will come up
>>             hot (completely cold instances for an initial deployment
>>             is less of a concern, because we deploy immutably and get
>>             a natural warm-up period while we have 200% capacity
>>             online for a cluster)
>>      2. It's currently not a problem if a VM doesn't exit completely
>>         due to a dangling non-daemon thread or hung shutdown hook
>>
>>     Being able to trigger assembly/verification via jcmd without
>>     exiting, would make this far easier for us to support. If the
>>     overhead of the instrumentation for CDS can be avoided, being 
>> able
>>     to take a snapshot at any time on any VM would be better still,
>>     but that wouldn't be an impediment for us: we'll know that the
>>     instance will be used for training at boot time.
>>
>>     We build nightlies of all the currently active OpenJDK projects,
>>     so if you land anything on premain between EA builds that you'd
>>     like us to try, let us know!
>>
>>     Cheers,
>>     Danny
>>
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