RFR (M): 8207266: ThreadMXBean::getThreadAllocatedBytes() can be quicker for self thread

Hohensee, Paul hohensee at amazon.com
Fri Aug 30 22:33:05 UTC 2019

Thanks for your review, Mandy. Revised webrev at http://cr.openjdk.java.net/~phh/8207266/webrev.02/.

I updated the CSR with your suggested javadoc for getCurrentThreadAllocatedBytes. It now matches that for getCurrentThreadUserTime and getCurrentThreadCputime. I also fixed the “convenient” -> “convenience” typos in j.l.m.ThreadMXBean.java.

I meant GetOneThreads to be the possessive, but don’t feel strongly either way so I’m fine with GetOneThread.

I updated ThreadImpl.java as you suggested, though in getThreadAllocatedBytes(long[] ids) I had to add a redundant-in-the-not-length-1-case check for a null ids reference.

Would someone take a look at the Hotspot side and the test please?


From: Mandy Chung <mandy.chung at oracle.com>
Date: Friday, August 30, 2019 at 10:22 AM
To: "Hohensee, Paul" <hohensee at amazon.com>
Cc: OpenJDK Serviceability <serviceability-dev at openjdk.java.net>, "hotspot-gc-dev at openjdk.java.net" <hotspot-gc-dev at openjdk.java.net>
Subject: Re: RFR (M): 8207266: ThreadMXBean::getThreadAllocatedBytes() can be quicker for self thread

OK.  That's better.  Some review comments:

The javadoc of getCurrentThreadAllocatedBytes() can simply say:

"Returns an approximation of the total amount of memory, in bytes,
allocated in heap memory for the current thread.

This is a convenient method for local management use and is equivalent
to calling getThreadAllocatedBytes(Thread.currentThread().getId()).


GetOneThreadsAllocatedMemory: s/OneThreads/OneThread/


  43     private static final String THREAD_ALLOCATED_MEMORY_NOT_SUPPORTED =
  44         "Thread allocated memory measurement is not supported.";

if (!isThreadAllocatedMemorySupported()) {
   throw new UnsupportedOperationException(THREAD_ALLOCATED_MEMORY_NOT_SUPPORTED);

Perhaps the above can be refactored as throwIfAllocatedMemoryUnsupported() method.

 391         if (ids.length == 1) {
 392             sizes[0] = -1;
 398             if (ids.length == 1) {
 399                 long id = ids[0];
 400                 sizes[0] = getThreadAllocatedMemory0(
 401                     Thread.currentThread().getId() == id ? 0 : id);
 402             } else {

It seems cleaner to handle the 1-element array case at the beginning
of this method:
   if (ids.length == 1) {
       long size = getThreadAllocatedBytes(ids[0]);
       return new long[] { size };

I didn't review the hotspot implementation and the test.

On 8/29/19 10:01 AM, Hohensee, Paul wrote:
My bad, Mandy. The webrev puts getCurrentThreadAllocatedBytes in com.sun.management.ThreadMXBean along with the current two getThreadAllocatedBytes methods for the reasons you list. I’ve updated the CSR to specify com.sun.management and added a rationale. AllocatedBytes is currently enabled by Hotspot by default because the overhead of recording TLAB occupancy is negligible.

There’s no new GC code, nor will there be, so imo we don’t have to involve the GC folks. I.e., the new JMM method GetOneThreadsAllocatedBytes uses the existing cooked_allocated_bytes JavaThread method, and getCurrentThreadAllocatedBytes is the same as getThreadAllocatedBytes: it just bypasses the thread lookup code.

I hadn’t tracked down what happens when getCurrentThreadUserTime and getCurrentThreadCpuTime are called before, but if I’m not mistaken, it the code in jcmd() in attachListener.cpp will call GetThreadCpuTimeWithKind in management.cpp, and it will ultimately use Thread::current() as the subject of the call, see os::current_thread_cpu_time in os_linux.cpp. That means that the CurrentThread methods should work remotely the same way they do locally. GetOneThreadsAllocatedBytes in management.cpp uses THREAD as its subject when called on behalf of getCurrentThreadAllocatedBytes, so it will also uses the current remote Java thread. Even if these methods only worked locally, there are many setups where apps are self-monitoring that could use the performance improvement.



From: Mandy Chung <mandy.chung at oracle.com><mailto:mandy.chung at oracle.com>
Date: Wednesday, August 28, 2019 at 3:59 PM
To: "Hohensee, Paul" <hohensee at amazon.com><mailto:hohensee at amazon.com>
Cc: OpenJDK Serviceability <serviceability-dev at openjdk.java.net><mailto:serviceability-dev at openjdk.java.net>, "hotspot-gc-dev at openjdk.java.net"<mailto:hotspot-gc-dev at openjdk.java.net> <hotspot-gc-dev at openjdk.java.net><mailto:hotspot-gc-dev at openjdk.java.net>
Subject: Re: RFR (M): 8207266: ThreadMXBean::getThreadAllocatedBytes() can be quicker for self thread

Hi Paul,

The CSR proposes this method in java.lang.management.ThreadMXBean as a Java SE feature.

Has this been discussed with the GC team to commit measuring current thread's allocated bytes as Java SE feature?   Can this be supported by all JVM implementation?   What is the overhead if this is enabled by default?  Does it need to be disabled?   This metric is from TLAB that might be okay.  This needs advice/discussion with GC experts.

I see that CSR mentions it can be disabled and link to isThreadAllocatedMemoryEnabled() and setThreadAllocatedMemoryEnabled() methods but these methods are defined in com.sun.management.ThreadMXBean.

As Alan points out, current thread makes sense only in local VM management.  When this is monitored from a JMX client (e.g. jconsole to connect to a running JVM, "currentThreadAllowcatedBytes" attribute is the current thread in jconsole process which invoking Thread::currentThread?

On 8/28/19 12:22 PM, Hohensee, Paul wrote:
Please review a performance improvement for ThreadMXBean.getThreadAllocatedBytes and the addition of getCurrentThreadAllocatedBytes.

JBS issue: https://bugs.openjdk.java.net/browse/JDK-8207266
Webrev: http://cr.openjdk.java.net/~phh/8207266/webrev.00/
CSR: https://bugs.openjdk.java.net/browse/JDK-8230311

Previous email threads:

The CSR is for adding ThreadMXBean.getCurrentThreadAllocatedBytes. I’d be great for someone to review it.

I took Mandy’s advice and put the fast paths in the library code. I added a new JMM method GetOneThreadsAllocatedBytes that works the same as GetThreadCpuTime: it uses a thread_id value of zero to distinguish the current thread. On my Mac laptop, the result runs 47x faster for the current thread than the old implementation.

The 3 tests in test/jdk/com/sun/management/ThreadMXBean all pass. I added code to ThreadAllocatedMemory.java to test getCurrentThreadAllocatedBytes as well as variations on getThreadAllocatedBytes(id). A submit repo job is in progress.



More information about the hotspot-gc-dev mailing list