NIF是什麼? A NIF library contains native implementation of some functions of an Erlang module.
不清楚的同學請參考http://www.erlang.org/doc/man/erl_nif.html.
這個功能對於擴充套件Erlang,利用現有的遺留c的財產,提高效能非常有幫助.
但是通常同學們會無視手冊裡面的一句話:

Avoid doing lengthy work in NIF calls as that may degrade the responsiveness of the VM. NIFs are called directly by the same scheduler thread that executed the calling Erlang code. The calling scheduler will thus be blocked from doing any other work until the NIF returns

導致了非常嚴重的設計問題. 比如在NIF裡面呼叫mysql client api, 作費時的IO操作等等, 我已經看到好幾個同學這麼幹了,為了揭示這個問題的嚴重性, davisp同學為我們寫了個例子來演示這個問題: 程式碼在這裡
https://github.com/davisp/sleepy.

Sleepy – A misbehaving NIF
This demonstrates what happens if a NIF takes a long time and is called from as many schedulers as exist in the VM. Namely, that the VM is halted until the NIF functions return.

我們來演示下把:

#  git clone https://github.com/davisp/sleepy.git
git clone https://github.com/davisp/sleepy.git
Initialized empty Git repository in /tmp/sleepy/.git/
remote: Counting objects: 7, done.
remote: Compressing objects: 100% (7/7), done.
remote: Total 7 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (7/7), done.
# cd sleepy
## 這個davisp同學是個Mac控, 我們在 linux下工作的, 需要修改下Makefile
# diff Makefile Makefile.orig 
2c2
< INCLUDES = -I$(OTPROOT)/erts-5.7.5/include/
---
> INCLUDES = -I$(OTPROOT)/erts-5.8.2/include/
5c5
< GCCFLAGS = -O3 -fPIC -bundle -flat_namespace -undefined suppress -fno-common -Wall 
---
> #GCCFLAGS = -O3 -fPIC -bundle -flat_namespace -undefined suppress -fno-common -Wall 
8c8
< #GCCFLAGS = -O3 -fPIC -shared -fno-common -Wall
---
> GCCFLAGS = -O3 -fPIC -shared -fno-common -Wall

##david同學寫的程式碼不夠體貼人
# diff sleepy.erl  sleepy.erl.orig 
6c6
<     erlang:load_nif("./sleepy", 0).
---
>     erlang:load_nif("sleepy", 0).

# 好吧, 編譯執行

# make run
erl -noshell -s lockvm lock -s init stop
Starting heartbeat.
Tick
Tick
Tick
Tick
Locking the VM 從這開始你的VM被堵死
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick
Tick

我們可以看到NIF在sleep, 堵死了所有的排程器, 你的erlang VM也在睡覺, zZzZZz, 好舒服哦, 你的系統性能就慘了.

結論: 避免在NIF裡面作費時的操作.

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