Timing bugs

Gary Adams gary.adams at oracle.com
Mon Nov 7 12:28:30 UTC 2011

  On 11/ 6/11 08:05 PM, David Holmes wrote:
> Hi Gary,
> On 4/11/2011 11:36 PM, Gary Adams wrote:
>> I've started to look at timing related bugs that have been open
>> for a while, but have not had sufficient priority to make it to the
>> top of the list of bugs to be fixed. Thought I'd start with some
>> low hanging fruit with simple bug fixes.
> Sometimes apparently low-hanging fruit is attached to the tree with steel wire 
> ;-) (and sometimes barbed wire at that)

Had a feeling when I pulled on the fruit the trap door
would open in the floor ...
>> 6731620: TEST_BUG: java/util/Timer/Args.java is too optimistic about the
>> execution time of System.out.printf
>> This seems like a simply problem to avoid two calls to get the current time
>> and to eliminated the time to process the print statement
>> from the evaluation of the test elapsed time.
>> Replacing this sequence ;
>> System.out.printf("elapsed=%d%n", System.currentTimeMillis() - start);
>> check(System.currentTimeMillis() - start < 500);
>> with
>> elapsed = System.currentTimeMillis() - start;
>> System.out.printf("elapsed=%d%n", elapsed);
>> check(elapsed < 500);
> This seems reasonable on the face of it. But a couple of other observations 
> regarding the test code:
> First the 500 is somewhat arbitrary - the test is preparing three timertasks 
> with initial expiry times in the past and expecting them to all execute 
> "immediately" (one multiple times). This immediacy is translated into 
> "completes within 500ms". These type of timing constants should be eradicated 
> if possible, or preferably made configurable if not. It is a difficult problem 
> to address if you want to try and detect something taking an unexpectedly long 
> time, but have no idea what your execution environment will be.

I think I'm signing up for making this one test reliable and
not taking on all of the bad timeout assumptions. The 500 millisecond
constraint seems reasonable in this case for the work expected to be
> Second, the current code won't detect a true failure that results in a hang - 
> the testcase will hang and presumably get timed-out by the test harness. It 
> might be better to apply a timeout to the await() calls instead and fail only 
> if they do timeout.

I thought the hung test was the responsibility of the test harness.
It makes for simpler individual tests if that responsibility is
delegated to the harness.
> Third, in this case it would also be prudent to (re-)read the start time after 
> the setup has been done ie after creating the countDownLatches. If this is run 
> in samevm mode or agentvm mode those constructions may trigger a full GC and 
> the ensuing delay could cause the test to again fail.

Sounds reasonable I'll add that change to the fix.
> That all said, I'd see combining your suggested fix with moving the point at 
> which start is measured as definite improvements in the test.
>> I plan to test the fix on a 300MHz linux/arm device.
>> I'll provide a proper webrev as soon as I have author rights
>> confirmed. I'm looking for reviewer and a committer,
>> once I get the fix tested locally.
> I presume by "committer" you mean somebody to actually do the push for you - I 
> think we generally refer to that as a "sponsor" (even though not part of 
> OpenJDK terminology). In any case that should be someone from TL group and 
> I'll let them respond further on that and provide any additional review comments.
> I'm happy to act as another Reviewer.

> Cheers,
> David
>> Thanks
>> Gary Adams

More information about the core-libs-dev mailing list