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

[David Holmes]

Hi Paul,

On 13/09/2019 10:29 am, Hohensee, Paul wrote:
> Thanks for clarifying the review rules. Would someone from the 
> serviceability team please review? New webrev at
> 
> http://cr.openjdk.java.net/~phh/8207266/webrev.07/

One aspect of the functional change needs clarification for me - and 
apologies if this has been covered in the past. It seems to me that 
currently we only check isThreadAllocatedMemorySupported for these 
operations, but if I read things correctly the updated code additionally 
checks isThreadAllocatedMemoryEnabled, which is a behaviour change not 
mentioned in the CSR.

> I didn’t disturb the existing checks in the test, just added code to 
> check the result of getThreadAllocatedBytes(long) on a non-current 
> thread, plus the back-to-back no-allocation checks. The former wasn’t 
> needed before because getThreadAllocatedBytes(long) was just a wrapper 
> around getThreadAllocatedBytes(long []). This patch changes that, so I 
> added a separate test. The latter is supposed to fail if there’s object 
> allocation on calls to getCurrentThreadAllocatedBytes and 
> getThreadAllocatedBytes(long). I.e., a feature, not a bug, because 
> accumulation of transient small objects can be a performance problem. 
> Thanks to your review, I noticed that the back-to-back check on the 
> current thread was using getThreadAllocatedBytes(long) instead of 
> getCurrentThreadAllocatedBytes and fixed it. I also removed all 
> instances of “TEST FAILED: “.

The back-to-back check is not valid in general. You don't know if the 
first check might trigger some class loading on the return path after it 
has obtained the first memory value. The check might also fail if using 
JVMCI and some compilation related activity occurs in the current thread 
on the second call. Also with the introduction of handshakes its 
possible the current thread might hit a safepoint checks that results in 
it executing a handshake operation that performs allocation. Potentially 
there could be numerous non-deterministic actions that might occur 
leading to unanticipated allocation.

I understand what you want to test here, I just don't think it is 
reliably doable.

Thanks,
David
-----

> 
> Paul
> 
> *From: *Mandy Chung <mandy.chung at oracle.com>
> *Date: *Thursday, September 12, 2019 at 10:09 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
> 
> On 9/3/19 12:38 PM, Hohensee, Paul wrote:
> 
>     Minor update in new webrevhttp://cr.openjdk.java.net/~phh/8207266/webrev.05/.
> 
> 
> I only reviewed the library side implementation that looks good.  I 
> expect the serviceability team to review the test and hotspot change.
> 
> 
>     Need a confirmatory review to push this. If I understand the rules correctly, it doesn't need a Reviewer review since Mandy's already reviewed it, it just needs a Committer review.
> 
> 
> You need another reviewer to advice the following because I was not 
> close to the ThreadsList work.
> 
> 2087   ThreadsListHandle tlh;
> 
> 2088   JavaThread* java_thread = tlh.list()->find_JavaThread_from_java_tid(thread_id);
> 
> 2089
> 
> 2090   if (java_thread != NULL) {
> 
> 2091     return java_thread->cooked_allocated_bytes();
> 
> 2092   }
> 
> This looks right to me.
> 
> test/jdk/com/sun/management/ThreadMXBean/ThreadAllocatedMemory.java
> 
> -                "ThreadAllocatedMemory is expected to be disabled");
> 
> +                "TEST FAILED: ThreadAllocatedMemory is expected to be 
> disabled");
> 
> Prepending "TEST FAILED" in exception message (in several places)
> 
> seems redundant since such RuntimeException is thrown and expected
> 
> a test failure.
> 
> +        // back-to-back calls shouldn't allocate any memory
> 
> +        size = mbean.getThreadAllocatedBytes(id);
> 
> +        size1 = mbean.getThreadAllocatedBytes(id);
> 
> +        if (size1 != size) {
> 
> Is there anything in the test can do to help guarantee this? I didn't
> 
> closely review this test.  The main thing I advice is to improve
> 
> the reliability of this test.  Put it in another way, we want to
> 
> ensure that this test change will pass all the time in various
> 
> test configuration.
> 
> Mandy
>