Question about mark words in allocated objects

[Doug Simon]

On Sep 16, 2013, at 7:01 PM, "Deneau, Tom" <tom.deneau at amd.com> wrote:

> We are experimenting with an HSA device doing object allocation.
> 
> In our prototype, there is one or more idle Java threads acting as a "donor threads", in that each donates its TLAB.
> In the simple case below, there is only one donor thread.
> 
> Since there can be more HSA device workitems than there are donor threads, the  workitems use atomic operations to bump the tlab.top pointer of a donor thread.  The usual graal code then takes over to fill in the contents of the allocated item, including the mark word, class pointer, etc.
> In our usual junit test cases, we run the "kernel function" sequentially on the CPU  and then as an HSA kernel on the GPU and compare results.  This is a "--vm server" run so the CPU side is not going thru graal
> 
> In my test case, I am using a JNI function to print out the full object contents including the header after the "kernel" completes.  The atomic pointer bumps seem to work correctly but I've noticed a slight difference in the header contents.
> 
> Since we are running with the default UseBiasedLocking enabled, I can see that when the object is initialized from the "prototype mark word", it is initialized with the value 5 (I assume this means anonymously biased?).

It means biasable but unlocked (see HotSpotReplacementsUtil.biasedLockPattern() and its usage in MonitorSnippets).

> I have noticed that when we run normal sequential Java and print out the mark word after the kernel has run, the mark word has a value of 1 (unlocked).  But if the kernel has run on the GPU and we print out the mark word, it is still 5 in each object.  The rest of the object contents matches between the CPU and GPU Runs.
> 
> Where does the mark word get changed from 5 to 1 on the cpu side?

This is almost certainly due to the delayed initialization of biased locking - see BiasedLocking::init (in biasedLocking.cpp). Try adding -XX:BiasedLockingStartupDelay=0 as a VM option.

-Doug