This document summarizes Erlang processes and scheduling in Erlang. It discusses the process control block, stack and heap, message passing, the scheduler, workload balancing across schedulers, and garbage collection. The scheduler prioritizes ports, maximum, high and normal/low priority queues. It will consume all reductions for a process or pause it waiting for a message. The garbage collector uses copying collection for process heaps and reference counting for shared binaries.

1. Erlang 调度初探
庄晓丹丹 2017/01/06

2. Ping-Pong

3. Ping-Pong
• ping_pong.ex
• Context Switch：1.x us on my machine(2.5 GHz
Intel Core i7).

4. Process Structure
• C Struct : erl_process.h
• PCB: process control block
• process id
• stack/heap position
• argument registers
• program counter
• stack
• private heap : compound data structures, such as tuples, lists etc.
• heap fragments: used when there is not enough free space in the heap and
garbage collection cannot be performed to get more free memory.

5. Erlang Process

6. Stack and Heap

7. Process
来⾃自： http://www.cnblogs.com/me-sa/archive/2011/11/13/
erlang0014.html

8. Message Passing
• send / receive
• Coping message
• Sends a message to it self, doesn’t copy.
• binary 拷⻉贝引⽤用
• < 64 : private heap
• >= 64 : 全局共享
• 推荐：⼩小消息，⼤大计算

9. Scheduler
• Reductions: 2000
• erl_vm.h :
• #define CONTEXT_REDS 2000 /* Swap process out after this number */
• erts_schedule in erl_process.c
• consumes all its reductions
• or pauses to wait for a message
• Four priorities: low | normal | high | max
• Four queues: ports, maximum, high, normal/low.
• ports > max > high > normal/low
• normal/low: skip a low priority process for a number of times
• Round-Robin in queue.

10. A Scheduler
图⽚片来⾃自 Google images

11. SMP
• Multiple processors => Multiple scheduler
• erl +S Schedulers:SchedulerOnline
• defaults to logical processors configured
• Balancing workload on multiple processors

12. SMP Schedulers
图⽚片来源：http://ithelp.ithome.com.tw/articles/10161709

13. Workload Balancing
• Check workload periodically by one scheduler
• Default period: 2000*2000 reductions.
• erl_process.c:
• #define ERTS_RUNQ_CHECK_BALANCE_REDS_PER_SCHED
(2000*CONTEXT_REDS)
• check_balance in erl_process.c
• During a period:
• work sharing/stealing
• determines the number of active schedulers for the next period based on
the load of the current period

15. Number of schedulers

16. Next active schedulers

17. Work migration

18. Memory Management
http://erlang.org/doc/man/erts_alloc.html

20. Garbage Collection
• Process heap: copying generational garbage
collector.
• 共享 Binary: 引⽤用计数
• https://www.erlang-solutions.com/blog/
erlang-19-0-garbage-collector.html

21. GC

22. Control GC
• erl +h min_heap_size
• spawn min_heap_size/min_bin_vheap_size
• :erlang.garbage_collect()
• Statistics:
• :erlang.process_info(pid, :garbage_collection)
• :erlang.system_info(:garbage_collection)

23. 参考资料料
• http://www.diva-portal.org/smash/get/diva2:392243/
FULLTEXT01.pdf
• https://www.erlang-solutions.com/blog/erlang-19-0-garbage-
collector.html
• https://hamidreza-s.github.io/erlang/scheduling/real-time/
preemptive/migration/2016/02/09/erlang-scheduler-details.html
• http://www.cnblogs.com/me-sa/archive/2011/11/13/erlang0014.html
• https://www.zhihu.com/question/24732869