Pauses when writing mmap-ed files

See my blog post for details.

1. Start a new Ubuntu image.
2. Run the following commands:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install linux-virtual gcc screen openjdk-8-jdk
sudo reboot

3. Copy these files to this machine.
4. gcc --std=gnu99 -O3 -Wall -Wextra -o mmapwritepause mmapwritepause.c
5. Start screen. In one terminal: ./mmapwritepause /tmp/mmapout
6. In another terminal: ./diskload.sh
7. Watch the output of mmapwritepause; it will print long pauses.

JVM reproduction

1. javac MakeGarbage.java
2. java -XX:+PrintGCDetails -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCApplicationStoppedTime -Xmx1G -Xms1G MakeGarbage
3. In another terminal, run ./diskload.sh

This will show large safepoint times, correlated with when mmapwritepause reports pauses. To verify that disabling the stats fixes the problem, add -XX:+PerfDisableSharedMem and rerun it.