Linux进程与线程
无论是Java还是其他语言，无论如何定义线程模型和实现，基于底层角度而言都要归属到操作系统层面上的线程（LWP：轻量级线程技术映射到了内核线程）概念就不提了。
Richard Stevens对线程的描述（原文）
fork is expensive. Memory is copied from the parent to the child, all descriptors are duplicated in the child, and so on. Current implementations use a technique called copy-on-write, which avoids a copy of the parent’s data space to the child until the child needs its own copy. But, regardless of this optimization, fork is expensive. IPC is required to pass information between the parent and child after the fork. Passing information from the parent to the child before the fork is easy, since the child starts with a copy of the parent’s data space and with a copy of all the parent’s descriptors. But, returning information from the child to the parent takes more work. Threads help with both problems. Threads are sometimes called lightweight processes since a thread is “lighter weight” than a process. That is, thread creation can be 10–100 times faster than process creation. All threads within a process share the same global memory. This makes the sharing of information easy between the threads, but along with this simplicity comes the problem.
Richard Stevens对线程的描述（中文）
Linux中创建进程用fork操作，线程用clone操作。
通过ps -ef 看到的是进程列表，线程可以通过ps -eLf来查看。
用top命令的话，通过H开关也可以切换到线程视图。
具体到Java线程模型，规范是没有规定Java线程和系统线程的对应关系的，不过目前常见的实现是一对一的。
参考
http://openjdk.java.net/groups/hotspot/docs/RuntimeOverview.html#Thread%20Management|outline
问题排查思路
如果创建不了Java线程，报错是
Exception in thread “main” java.lang.OutOfMemoryError: unable to create new native thread
下面是常见的问题原因
内存太小
在Java中创建一个线程需要消耗一定的栈空间，默认的栈空间是1M(可以根据应用情况指定-Xss参数进行调整)，栈空间过小或递归调用过深，可能会出现StackOverflowError。
对于一个进程来说，假设一定量可使用的内存，分配给堆空间的越多，留给栈空间的就越少。
-
这个限制常见于32位Java应用，进程空间4G，用户空间2G(Linux下3G，所以通常堆可以设置更大些)。
减去堆空间大小(通过-Xms、-Xmx指定范围)
减去非堆空间(其中永久代部分通过PermSize、MaxPermSize指定大小，在Java8换成了MetaSpace，默认不限制大小)。
再减去虚拟机自身消耗。
剩下的就是栈空间，假设剩下300M，那么理论上就限制了只能开300线程（-Xss1M）。
不过对于64位应用，由于进程空间近乎无限大，所以可以不考虑这个问题。
ulimit限制
线程数还会受到系统限制，系统限制通过ulimit -a可以查看到。
https://ss64.com/bash/ulimit.html
caixj@Lenovo-PC:~$ ulimit -a
core file size (blocks, -c) 0
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 7823
max locked memory (kbytes, -l) 64
max memory size (kbytes, -m) unlimited
open files (-n) 1024
pipe size (512 bytes, -p) 8
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 8192
cpu time (seconds, -t) unlimited
max user processes (-u) 7823
virtual memory (kbytes, -v) unlimited
file locks (-x) unlimited
相关的限制有
max memory size ：最大内存限制，在64位系统上通常都设置成unlimited
max user processes ： 每用户总的最大进程数(包括线程)
virtual memory - 虚拟内存限制，在64位系统上通常都设置成unlimited
这些参数可以通过ulimit命令(当前用户临时生效)或者配置文件/etc/security/limits.conf(永久生效)进行修改。检查某个进程的限制是否生效，可以通过/proc/PID/limits查看运行时状态。
参数sys.kernel.threads-max限制
https://www.kernel.org/doc/Documentation/sysctl/kernel.txt
This value controls the maximum number of threads that can be created
using fork().
During initialization the kernel sets this value such that even if the
maximum number of threads is created, the thread structures occupy only
a part (1/8th) of the available RAM pages.
The minimum value that can be written to threads-max is 20.
The maximum value that can be written to threads-max is given by the
constant FUTEX_TID_MASK (0x3fffffff).
If a value outside of this range is written to threads-max an error
EINVAL occurs.
The value written is checked against the available RAM pages. If the
thread structures would occupy too much (more than 1/8th) of the
available RAM pages threads-max is reduced accordingly.
表示系统全局的总线程数限制。设置方式有:
运行时限制,临时生效
echo 999999 > /proc/sys/kernel/threads-max
修改/etc/sysctl.conf，永久生效
sys.kernel.threads-max = 999999
参数sys.kernel.pid_max限制
https://www.kernel.org/doc/Documentation/sysctl/kernel.txt
PID allocation wrap value. When the kernel's next PID value
reaches this value, it wraps back to a minimum PID value.
PIDs of value pid_max or larger are not allocated.
表示系统全局的PID号数值的限制。设置方式有:
运行时限制,临时生效
echo 999999 > /proc/sys/kernel/pid_max
修改/etc/sysctl.conf，永久生效
sys.kernel.pid_max = 999999
参数sys.vm.max_map_count限制
https://www.kernel.org/doc/Documentation/sysctl/vm.txt
This file contains the maximum number of memory map areas a process
may have. Memory map areas are used as a side-effect of calling
malloc, directly by mmap, mprotect, and madvise, and also when loading
shared libraries.
While most applications need less than a thousand maps, certain
programs, particularly malloc debuggers, may consume lots of them,
e.g., up to one or two maps per allocation.
The default value is 65536.
表示单个进程所能使用内存映射空间的数量限制。设置方式有:
方式1 运行时限制,临时生效
echo 999999 > /proc/sys/vm/max_map_count
方式2 修改/etc/sysctl.conf，永久生效
sys.vm.max_map_count = 999999
在其他资源可用的情况下，单个vm能开启的最大线程数是这个值的一半，可以通过
cat /proc/PID/maps | wc -l查看目前使用的映射数量。
至于为什么只有一半，结合一些材料和源码分析了一下:
常见的警告信息是这样的，见
JavaThread::create_stack_guard_pages()
Attempt to protect stack guard pages failed.
Attempt to deallocate stack guard pages failed.
见current_stack_region()的图示，结合一下R大的相关解
释:http://hllvm.group.iteye.com/group/topic/37717
如下所示，通常的Java线程，会包括一个glibc的guard page和HotSpot的guard pages，其中JavaThread::create_stack_guard_pages()就是创建HotSpot Guard Pages用的，这里正常应该会有2次VMA，所以最大值只能有一半，从/proc/PID/maps中也可以看到增加一个线程会增加2个地址相连的映射空间。
// Java thread:
//
// Low memory addresses
// +------------------------+
// | |\ JavaThread created by VM does not have glibc
// | glibc guard page | - guard, attached Java thread usually has
// | |/ 1 page glibc guard.
// P1 +------------------------+ Thread::stack_base() - Thread::stack_size()
// | |\
// | HotSpot Guard Pages | - red and yellow pages
// | |/
// +------------------------+ JavaThread::stack_yellow_zone_base()
// | |\
// | Normal Stack | -
// | |/
// P2 +------------------------+ Thread::stack_base()
//
// Non-Java thread:
//
// Low memory addresses
// +------------------------+
// | |\
// | glibc guard page | - usually 1 page
// | |/
// P1 +------------------------+ Thread::stack_base() - Thread::stack_size()
// | |\
// | Normal Stack | -
// | |/
// P2 +------------------------+ Thread::stack_base()
//
// ** P1 (aka bottom) and size ( P2 = P1 - size) are the address and stack size returned from
// pthread_attr_getstack()
