Hi Paul,
Good to see this getting in. Cleaner API depends on it ;-)
On 11/23/2015 05:38 PM, Paul Sandoz wrote:
Hi,
Please review the addition of Reference.reachabilityFence contributed by
Aleksey, Doug and myself:
http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8133348-reachability-fence-jdk/webrev/
<http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8133348-reachability-fence-jdk/webrev/>
http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8133348-reachability-fence-hotspot/webrev/
<http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8133348-reachability-fence-hotspot/webrev/>
The implementation approach marks the method Reference.reachabilityFence as not
inline-able, thereby “keeping alive” an object passed to the method until at
least after the method call.
The testing approach i have taken is to verify that the method does not get
inlined either in C1 or C2. The test approach seems fragile (as fragile as the
accessor-based test i code-cargo-culted from) but passes ok through JPRT.
I could not find a suitable mechanism in WhiteBox. Is there a more reliable
mechanism to determine what methods are inlined into a compiled method?
There is another testing approach in the VarHandles sandbox:
http://hg.openjdk.java.net/jdk9/sandbox/jdk/rev/433114b32d2d#l2.2
<http://hg.openjdk.java.net/jdk9/sandbox/jdk/rev/433114b32d2d#l2.2>
But i am not confident that the test can be run within a reasonable time and
reliably on all platforms and VM modes.
Paul.
The problem of this other approach is that you constantly invoke
System.gc(), so your time to warm-up the code to get it compiled is big.
Why don't you allow the loop to run faster for some warm-up time and
kick GC in only after that. Like:
http://cr.openjdk.java.net/~plevart/misc/ReachabilityFence/ReachabilityFence.java
This works reliably on Linux and only takes not much more than 200ms per
test.
You are also relying on JIT to do on-stack replacement. It seems to work
for all modes (besides interpreter-only).
Regards, Peter
