RFR(m): 8223306: Remove threads linked list (use ThreadsList's array in SA)

Daniel D. Daugherty daniel.daugherty at oracle.com
Wed May 8 16:02:00 UTC 2019 

On 5/8/19 5:20 AM, Robbin Ehn wrote:
> Correct links:
> http://cr.openjdk.java.net/~rehn/8223306/v2/webrev/src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/CommandProcessor.java.sdiff.html 
>
>
> http://cr.openjdk.java.net/~rehn/8223306/v2/inc/webrev/
> http://cr.openjdk.java.net/~rehn/8223306/v2/webrev/

src/hotspot/share/runtime/thread.hpp
     No comments.

src/hotspot/share/runtime/thread.cpp
     No comments.

src/hotspot/share/runtime/threadSMR.hpp
     No comments.

src/hotspot/share/runtime/vmStructs.cpp
     No comments.

It's nice to see that no code is using the old Threads::_thread_list
or JavaThread::next() anymore. I have a vague memory of some Compiler
Thread code still using it back when we first layered in ThreadSMR...
Something about a fast way to find the next CompilerThread...
That must have changed.

Onward...

General comment - Please make sure to update all copyright years before
                   pushing this changeset.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/CommandProcessor.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/bsd/BsdDebuggerLocal.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/ObjectHeap.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/runtime/DeadlockDetector.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/runtime/JavaThread.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/runtime/Threads.java
     L46:
     L47:     private static AddressField      threadsField;
     L48:     private static CIntegerField     lengthField;
         nit - please delete blank line on L46
         nit - please reduce the space between type and variable names
               (I have no preference if you still keep them aligned)

     nit - Please delete blank line on L74.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/tools/PStack.java
     old L203:       Threads threads = VM.getVM().getThreads();
     old L204:       for (JavaThread cur = threads.first(); cur != null; 
cur = cur.next()) {
     new L203:       VM.getVM().getThreads().doJavaThreads((cur) -> {
         In this case, you did a lambda conversion...

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/tools/StackTrace.java
     old L75:             for (JavaThread cur = threads.first(); cur != 
null; cur = cur.next(), i++) {
     new L75:             for (int k = 0; k < 
threads.getNumberOfThreads(); k++) {
     new L76:                 JavaThread cur = threads.getJavaThreadAt(k);
         In this case, you didn't do a lambda conversion...

         I'm trying to grok a reason for the different styles...
         Update: Is is maybe a control flow thing? (No, I don't know much
            about lambdas.) As in: Loops that "break" or early return are
            not amenable to conversion to a lambda... (just guessing)

     L74:             int i = 1;
         Not your bug, but I think that 'i' is not used.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/ui/JavaThreadsPanel.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/utilities/AbstractHeapGraphWriter.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/utilities/HeapHprofBinWriter.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/utilities/PointerFinder.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/utilities/ReversePtrsAnalysis.java
     No comments.

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/utilities/soql/JSJavaVM.java
     No comments.

test/hotspot/jtreg/serviceability/sa/ClhsdbField.java
     No comments.

test/hotspot/jtreg/serviceability/sa/ClhsdbPrintStatics.java
     No comments.

test/hotspot/jtreg/serviceability/sa/ClhsdbVmStructsDump.java
     No comments.

This all looks good to me... so thumbs up!

I have some reservations about using lambdas in a debugging tool.
My personal philosophy about debugging tools is that they should
be built on the simplest/most stable technology to reduce the
chances of the more complicated technology failing during a debug
session. I hate it when my debugger crashes!

That said, SA is pretty much standalone so use of lambdas in this
debugging tool shouldn't affect the JVM or core file being debugged.

Again, thumbs up!

Dan


>
> /Robbin
>
> On 2019-05-08 11:17, Robbin Ehn wrote:
>> Hi David,
>>
>> I changed to normal for:
>> http://rehn-ws.se.oracle.com/cr_mirror/8223306/v2/webrev/src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/CommandProcessor.java.sdiff.html 
>>
>>
>> Full:
>> http://rehn-ws.se.oracle.com/cr_mirror/8223306/v2/webrev/
>> Inc:
>> http://rehn-ws.se.oracle.com/cr_mirror/8223306/v2/inc/webrev/
>>
>> Passes t1-2
>>
>> Thanks, Robbin
>>
>>
>> On 2019-05-07 09:47, David Holmes wrote:
>>> Hi Robbin,
>>>
>>> On 7/05/2019 4:50 pm, Robbin Ehn wrote:
>>>> Hi David,
>>>>
>>>> On 5/7/19 12:40 AM, David Holmes wrote:
>>>>> Hi Robbin,
>>>>>
>>>>> I have a few concerns here.
>>>>>
>>>>> First I can't see how you are actually integrating the SA with the 
>>>>> ThreadSMR. You've exposed the _java_thread_list for it to iterate 
>>>>> but IIRC that list can be updated when threads are added/removed 
>>>>> and I'm not seeing how the SA is left iterating a valid list - 
>>>>> we'd normally using a ThreadsListHandle for that ?? (I may need a 
>>>>> refresher on how this list is actually maintained.)
>>>>
>>>> The processes must be paused. If the processes would be running the 
>>>> linked list is also broken since if we unlink and delete a 
>>>> JavaThread and then later SA follows that _next pointer.
>>>
>>> Ah good point. Thanks for clarifying.
>>>
>>>>>
>>>>> The conversion from external iteration of the list (the for loop) 
>>>>> to internal iteration (passing a lambda to JavaThreadsDo) is also 
>>>>> problematic. First I'd be very wary about introducing lambda 
>>>>> expressions into the SA code - lambda drags in a lot of supporting 
>>>>> code that could have an impact on the way SA functions. There are 
>>>>> places where we have to avoid lambdas due to 
>>>>> bootstrapping/initialization issues and I think the SA may be an 
>>>>> area where we also want to keep the code extremely simple.
>>>>
>>>> There are already several usages of lambdas in SA code, e.g. 
>>>> LinuxDebuggerLocal.java. SA is not a core module, it's an 
>>>> application, there is not a bootstrap issue or so.
>>>
>>> Hmm okay. Lambda carries a lot of baggage. But if its already being 
>>> used ...
>>>
>>>>> Second by using lambda's with internal iteration you've lost the 
>>>>> ability to terminate the iteration loop! In the existing code if 
>>>>> we have a return in the for-loop then we not only terminate the 
>>>>> loop but the enclosing method. With the lambda the "return" just 
>>>>> ends the current iteration and JavaThreadsDo will then continue 
>>>>> with the next thread - so we don't even terminate the iteration 
>>>>> let alone the method performing the iteration. So for places were 
>>>>> we want to process one thread, for example, we will continue to 
>>>>> iterate all remaining threads and just do nothing with them. 
>>>>> That's very inefficient.
>>>>
>>>> That's why I only used the internal iteration where we didn't have 
>>>> early returns.
>>>
>>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/CommandProcessor.java 
>>> - original code:
>>>
>>> 1556         new Command("where", "where { -a | id }", false) {
>>> 1557             public void doit(Tokens t) {
>>> ...
>>> 1564                     for (JavaThread thread = threads.first(); 
>>> thread != null; thread = thread.next()) {
>>> 1565                         ByteArrayOutputStream bos = new 
>>> ByteArrayOutputStream();
>>> 1566                         thread.printThreadIDOn(new 
>>> PrintStream(bos));
>>> 1567                         if (all || bos.toString().equals(name)) {
>>> 1568                             out.println("Thread " + 
>>> bos.toString() + " Address: " + thread.getAddress());
>>> ...
>>> 1577                             }
>>> 1578                             if (!all) return;
>>>
>>> That looks like an early return to me.
>>>
>>> Cheers,
>>> David
>>> -----
>>>
>>>
>>>> Thanks, Robbin
>>>>
>>>>>
>>>>> Thanks,
>>>>> David
>>>>>
>>>>> On 6/05/2019 5:31 pm, Robbin Ehn wrote:
>>>>>> Hi, please review.
>>>>>>
>>>>>> Old threads linked list remove and updated SA to use ThreadsList 
>>>>>> array instead.
>>>>>>
>>>>>> Issue:
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8223306
>>>>>> Webrev:
>>>>>> http://cr.openjdk.java.net/~rehn/8223306/webrev/
>>>>>>
>>>>>> Passes t1-3 (which includes SA tests).
>>>>>>
>>>>>> Thanks, Robbin

More information about the hotspot-runtime-dev mailing list