RFR: 8146705: Improve JVMCI support for blocking compilation

[Igor Veresov]

Makes sense. Looks good to me.

igor

> On Jan 11, 2016, at 5:18 AM, Doug Simon <doug.simon at oracle.com> wrote:
> 
> The CompileBroker currently uses a simple timeout of 1 second when waiting for a blocking JVMCI compilation to complete. This approach is too simple. JVMCI compiler threads themselves flood the compilation queues with compilation requests; such compilations cannot be blocking (the JVMCI compiler can easily cause the system to deadlock). This flooding means that application submitted tasks often timeout before the tasks even start compiling. 
> Once a JVMCI thread starts compiling a task, there is still the risk of it deadlocking. The current timeout mechanism needs to be augmented with a test of the compiler thread's state. As long as it's not blocked for too long, we know the compiler is making progress and will eventually complete.
> 
> This review is for changes that address the above issues as follows: 
> 
> 1. Non-blocking tasks are selected before blocking tasks from the compilation queue. 
> 2. A thread waiting for a compilation task to complete checks the state of the compiler thread periodically (500ms intervals). If 5 successive checks see a blocked thread, the compilation times out and the waiting thread is unblocked.
> 
> https://bugs.openjdk.java.net/browse/JDK-8146705
> http://cr.openjdk.java.net/~dnsimon/8146705/
> 
> -Doug