ADDITIONAL SYSTEM INFORMATION :
This does seem to be somewhat hardware dependent - some VMs don't exhibit the problem, and we haven't seen this on Windows yet. We've seen this on Linux 64-bit Openstack VMs, in at least 3 different setups.

A DESCRIPTION OF THE PROBLEM :
We have observed a significant performance regression in the Runtime.availableProcessors native API call.

On JDK versions prior to 1.8b191, a simple program can achieve upwards of 4 million calls to Runtime.availableProcessors a second on our test hardware.
On JDK build 1.8b191 and all later major and minor versions, including 11, the maximum it can achieve is around 40,000 calls a second - a performance drop of 100x.

This causes performance issues for CompletableFuture.waitingGet, which calls Runtime.availableProcessors in a tight loop as part of its spinwait - which is how we originally found the issue, as our application was exhibiting significant performance problems in our asynchronous code.

REGRESSION : Last worked in version 8u192

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached program on JDK version 1.8b182, and 1.8b191, on a 64bit Linux machine.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The performance should be the same
ACTUAL -
The performance drops by a factor of 100.

---------- BEGIN SOURCE ----------
    public static void main(String[] args) throws Exception {
        AtomicBoolean stop = new AtomicBoolean();
        AtomicInteger count = new AtomicInteger();

        new Thread(() -> {
            while (!stop.get()) {
                Runtime.getRuntime().availableProcessors();
                count.incrementAndGet();
            }
        }).start();

        try {
            int lastCount = 0;
            while (true) {
                Thread.sleep(1000);
                int thisCount = count.get();
                System.out.printf("%s calls/sec%n", thisCount - lastCount);
                lastCount = thisCount;
            }
        }
        finally {
            stop.set(true);
        }
    }
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None possible.

FREQUENCY : often