系统技术非业余研究

我们先看下man erl

+T Level:

Enables modified timing and sets the modified timing level. Currently valid range is 0-9. The timing of the runtime system will
change. A high level usually means a greater change than a low level. Changing the timing can be very useful for finding timing
related bugs.

Currently, modified timing affects the following:

Process spawning:
A process calling spawn, spawn_link, spawn_monitor, or spawn_opt will be scheduled out immediately after completing the
call. When higher modified timing levels are used, the caller will also sleep for a while after being scheduled out.

Context reductions:
The amount of reductions a process is a allowed to use before being scheduled out is increased or reduced.

Input reductions:
The amount of reductions performed before checking I/O is increased or reduced.

NOTE: Performance will suffer when modified timing is enabled. This flag is only intended for testing and debugging. Also note
that return_to and return_from trace messages will be lost when tracing on the spawn BIFs. This flag may be removed or changed
at any time without prior notice.

我们可以知道这个选项是用于修改VM的进程和IO调度的时间，以及延迟spawn的执行时间， 使得和时间相关的问题，容易得到暴露，达到发现问题的目的。 但是Level是如何对于参数的呢？下面这个表格能回答问题了.
erts/emulator/beam/erl_init.c

 132ErtsModifiedTimings erts_modified_timings[] = {                                                    
 133    /* 0 */     {make_small(0), CONTEXT_REDS, INPUT_REDUCTIONS},                            
 134    /* 1 */     {make_small(0), 2*CONTEXT_REDS, 2*INPUT_REDUCTIONS},                               
 135    /* 2 */     {make_small(0), CONTEXT_REDS/2, INPUT_REDUCTIONS/2},                               
 136    /* 3 */     {make_small(0), 3*CONTEXT_REDS, 3*INPUT_REDUCTIONS},                               
 137    /* 4 */     {make_small(0), CONTEXT_REDS/3, 3*INPUT_REDUCTIONS},                               
 138    /* 5 */     {make_small(0), 4*CONTEXT_REDS, INPUT_REDUCTIONS/2},                               
 139    /* 6 */     {make_small(1), CONTEXT_REDS/4, 2*INPUT_REDUCTIONS},                               
 140    /* 7 */     {make_small(1), 5*CONTEXT_REDS, INPUT_REDUCTIONS/3},                               
 141    /* 8 */     {make_small(10), CONTEXT_REDS/5, 3*INPUT_REDUCTIONS},                              
 142    /* 9 */     {make_small(10), 6*CONTEXT_REDS, INPUT_REDUCTIONS/4}                               
 143};                                                                                                 
 

注意延时是以毫秒为单位的。

比如说: erl +T 8 那么spawn延时10ms, IO处理时间加大到3倍， 有利于快速处理IO事件。