系统技术非业余研究

Erlang很关键的一个能力就是软实时，可以显著提高应用的QOS。为什么可以做到软实时呢？参看这篇译文。这里面有二个东西没处理好会破坏erlang的公平调度机制：
1. BIF，靠trap机制来让出执行。
2. NIF，靠减少reductions来让出执行。

这二个机制都运行用户自己写的c代码来扩展erlang vm的功能。这些代码是跑在虚拟机的调度线程里头的，一旦每次处理太多东西，或者死锁什么的，会阻塞调度器,导致VM挂起，问题还是比较严重的。

erlang在最新的R16B03的版本中，很贴心的提供了long_schedule监控，让用户来提前发现这个问题并且解决这个问题。我摘抄下long_schedule的描述：

erlang:system_monitor(Arg) -> MonSettings

{long_schedule, Time}
If a process or port in the system runs uninterrupted for at least Time wall clock milliseconds, a message {monitor, PidOrPort, long_schedule, Info} is sent to MonitorPid. PidOrPort is the process or port that was running and Info is a list of two-element tuples describing the event. In case of a pid(), the tuples {timeout, Millis}, {in, Location} and {out, Location} will be present, where Location is either an MFA ({Module, Function, Arity}) describing the function where the process was scheduled in/out, or the atom undefined. In case of a port(), the tuples {timeout, Millis} and {port_op,Op} will be present. Op will be one of proc_sig, timeout, input, output, event or dist_cmd, depending on which driver callback was executing. proc_sig is an internal operation and should never appear, while the others represent the corresponding driver callbacks timeout, ready_input, ready_output, event and finally outputv (when the port is used by distribution). The Millis value in the timeout tuple will tell you the actual uninterrupted execution time of the process or port, which will always be >= the Time value supplied when starting the trace. New tuples may be added to the Info list in the future, and the order of the tuples in the list may be changed at any time without prior notice.

This can be used to detect problems with NIF’s or drivers that take too long to execute. Generally, 1 ms is considered a good maximum time for a driver callback or a NIF. However, a time sharing system should usually consider everything below 100 ms as “possible” and fairly “normal”. Schedule times above that might however indicate swapping or a NIF/driver that is misbehaving. Misbehaving NIF’s and drivers could cause bad resource utilization and bad overall performance of the system.

github上的提交参看这里，里面的testcase很好的演示了这点。

小结：system monitor能发现好多vm 潜在的问题，需要多挖掘。

祝玩得开心！