linux进程与线程的区别

个人理解，仅供自己参考。

本文参考

/article/5807250.html

/article/10290768.html

linux内核修炼之道

linux中，进程（process）是具有独立空间，process之间可以互不干扰地运行的基本单位。它们之间通过进程间通讯（IPC）传递数据，除此之外，不会有任何干扰，所以安全性好。而线程（Thread）是进程里面的，用来调度procedures的基本单位，它是：进程里面的一个负责处理某个步骤的基本单位。所以，线程是进程的一部分。

那为啥要区分进程和线程？其实，一开始没有线程这个概念，最早的工程师发现进程间通讯有一定缺陷，需要弥补，所以才改进了进程，把进程这个整体分块了！进程的缺陷是什么呢？

1.进程间通讯复杂，且CPU开销大。如果只是做简单的1+1运算，最好不要劳烦进程间通讯了，最好进程里面可以自己计算。

2.用fork()函数产生一个子进程，内存和CPU开销相当大，因为有大量的数据要copy。(为了有直观的感受，请参照本文结尾的代码，我和我的小伙伴都惊呆了)

这时，会产生疑问，开销大是个什么概念？我们看看进程和线程的数据结构：





再来看看线程Thread是怎么藏在进程Process里的



：


《程序员自我修养》中截图，展示了“进程内”的多线程，“进程空间”是给这个线程用的独立空间，给这个进程用的，不与线程共享。代码/数据/堆和打开的文件是可以共享的。



可见，Thread只需要堆栈指针、程序计数寄存器、一些优先级属性、等待序列和阻塞信号、thread自身的数据。

而Process的组成必须包括以下多出的部分：进程、进程群、用户、群组信息，环境，工作目录，Heap（堆，这个要很大空间），文件描述符，共享库，IPC工具。

因此，process内存开销大，通讯复杂的特点，使得工程师们想出了Thread这个东西。Process的开销是Thread的30倍。

Thread可以节省内存空间，创建简单，通讯简单，因此可以快速反应，处理速度可以提升10-100倍。但是因为Thread之间不是独立的，共用堆里面的数据，很可能产生冲突，所以thread要考虑同步的问题，不然数据会不安全。

自己的理解

什么是程序

程序是存储在存储介质上的可执行文件，是代码和数据的集合，程序装载进内存后，可以执行，是程序的动态实体。实体是个什么东西？自己体会吧。

什么是进程？进程对应着程序，程序是静态的代码，而进程就是这个代码实例化，动态化后，转成二进制在内存和CPU里面跑的程序，它有独立的空间，多个进程间需要特殊的IPC工具才能通讯。进程不但是数据和代码的集合，还可以打开文件，挂起信号，管理和保存堆栈信息等，这部分功能就是进程的动态特征。可见，它是系统分配内存资源的最小单位。相对于线程，不会再给它分配内存资源，线程只是在进程这个房间里面跑，利用进程已经获得的资源，所以说，进程才是系统分配资源的最小单位，而线程不是。

那线程是什么？一个进程可以分为两个部分：线程集合和资源集合。线程是进程的动态对象，是一组独立的指令流，多个线程共享进程中的资源。所有进程都拥有至少一个线程。相对于进程是资源管理的最小单位，线程是程序执行的最小单位。这样理解吧，程序有很多函数，我们可以按顺序地执行完，这样相当于单进程单线程。但我们写程序，函数经常是可以并行运作的，所以我们用线程这种技术，独立地运行可以并行操作的函数。就因为这种进程里面的并行操作，我们定义了最小的程序执行单位，那就是线程。

反过来想，如果进程是最小的程序执行单位，那会什么情况？相当于一个进程执行所有函数，就没有了并发处理的概念了嘛，处理速度就上不去。

重点：从linux内核来看，并没有线程和进程之分，没有线程的数据结构，linux内核里，线程只是共用数据空间的进程，这样的线程也称为轻量级的进程。参考最下面的进程代码，里面有thread_group数据链表。 同一进程的线程就被存放在这个链表中。线程是用户空间的概念，Thread到轻量级process的转换是通过POSIX库实现的，对应项目为：LinuxThreads.

说到最后，其实process和thread的区别很简单，thread就是为了节省空间，通过thread之间共用数据和堆栈，减少处理数据时候的数据copy，即减少了数据通讯，从而既提高内存使用效率，又减少了CPU的调度任务，从而提升了处理速度。

请想想，其实process和thread在抽象层面是超级简单的东西，只是我们不熟悉它的数据结构，操作流程，所以觉得好难区分。

我把它的抽象过程类比为如下：

一个人代表一个thread，一个办公室就是一个process，多个人在一个办公室就是多线程，一个人在一个办公室工作就是单线程。

假设一个公司有这样一个部门：他们本来一个人一个办公室，这样就要求他们之间通讯要发邮件，打电话。但是，有一天，他们发现这样效率很低，例如我想给某位大神看一个高清视频，我还得copy到u盘，然后拿到它办公室，它再copy下来看。这时，他们灵机一动，不如把办公室扩大一点，1个组一个办公室吧，反正大家的工作是密切相关的。 这样，有什么邮件、图片、视频、开发工具，大家从座位上移步就可以看了，不需要走出办公室。这样，这个公司就从没有线程的概念，发展到有多线程概念了。

这个例子好像很弱智，但不就是thread产生的原理嘛。

进程的数据结构代码

include/linux/sched.h

struct task_struct {
volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
void *stack;
atomic_t usage;
unsigned int flags;	/* per process flags, defined below */
unsigned int ptrace;

int lock_depth;		/* BKL lock depth */

#ifdef CONFIG_SMP
#ifdef __ARCH_WANT_UNLOCKED_CTXSW
int oncpu;
#endif
#endif

int prio, static_prio, normal_prio;
unsigned int rt_priority;
const struct sched_class *sched_class;
struct sched_entity se;
struct sched_rt_entity rt;

#ifdef CONFIG_PREEMPT_NOTIFIERS
/* list of struct preempt_notifier: */
struct hlist_head preempt_notifiers;
#endif

/*
* fpu_counter contains the number of consecutive context switches
* that the FPU is used. If this is over a threshold, the lazy fpu
* saving becomes unlazy to save the trap. This is an unsigned char
* so that after 256 times the counter wraps and the behavior turns
* lazy again; this to deal with bursty apps that only use FPU for
* a short time
*/
unsigned char fpu_counter;
#ifdef CONFIG_BLK_DEV_IO_TRACE
unsigned int btrace_seq;
#endif

unsigned int policy;
cpumask_t cpus_allowed;

#ifdef CONFIG_TREE_PREEMPT_RCU
int rcu_read_lock_nesting;
char rcu_read_unlock_special;
struct rcu_node *rcu_blocked_node;
struct list_head rcu_node_entry;
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */

#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
struct sched_info sched_info;
#endif

struct list_head tasks;
struct plist_node pushable_tasks;

struct mm_struct *mm, *active_mm;
#if defined(SPLIT_RSS_COUNTING)
struct task_rss_stat	rss_stat;
#endif
/* task state */
int exit_state;
int exit_code, exit_signal;
int pdeath_signal;  /*  The signal sent when the parent dies  */
/* ??? */
unsigned int personality;
unsigned did_exec:1;
unsigned in_execve:1;	/* Tell the LSMs that the process is doing an
* execve */
unsigned in_iowait:1;

/* Revert to default priority/policy when forking */
unsigned sched_reset_on_fork:1;

pid_t pid;
pid_t tgid;

#ifdef CONFIG_CC_STACKPROTECTOR
/* Canary value for the -fstack-protector gcc feature */
unsigned long stack_canary;
#endif

/*
* pointers to (original) parent process, youngest child, younger sibling,
* older sibling, respectively.  (p->father can be replaced with
* p->real_parent->pid)
*/
struct task_struct *real_parent; /* real parent process */
struct task_struct *parent; /* recipient of SIGCHLD, wait4() reports */
/*
* children/sibling forms the list of my natural children
*/
struct list_head children;	/* list of my children */
struct list_head sibling;	/* linkage in my parent's children list */
struct task_struct *group_leader;	/* threadgroup leader */

/*
* ptraced is the list of tasks this task is using ptrace on.
* This includes both natural children and PTRACE_ATTACH targets.
* p->ptrace_entry is p's link on the p->parent->ptraced list.
*/
struct list_head ptraced;
struct list_head ptrace_entry;

/*
* This is the tracer handle for the ptrace BTS extension.
* This field actually belongs to the ptracer task.
*/
struct bts_context *bts;

/* PID/PID hash table linkage. */
struct pid_link pids[PIDTYPE_MAX];
struct list_head thread_group;

struct completion *vfork_done;		/* for vfork() */
int __user *set_child_tid;		/* CLONE_CHILD_SETTID */
int __user *clear_child_tid;		/* CLONE_CHILD_CLEARTID */

cputime_t utime, stime, utimescaled, stimescaled;
cputime_t gtime;
#ifndef CONFIG_VIRT_CPU_ACCOUNTING
cputime_t prev_utime, prev_stime;
#endif
unsigned long nvcsw, nivcsw; /* context switch counts */
struct timespec start_time; 		/* monotonic time */
struct timespec real_start_time;	/* boot based time */
/* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */
unsigned long min_flt, maj_flt;

struct task_cputime cputime_expires;
struct list_head cpu_timers[3];

/* process credentials */
const struct cred *real_cred;	/* objective and real subjective task
* credentials (COW) */
const struct cred *cred;	/* effective (overridable) subjective task
* credentials (COW) */
struct mutex cred_guard_mutex;	/* guard against foreign influences on
* credential calculations
* (notably. ptrace) */
struct cred *replacement_session_keyring; /* for KEYCTL_SESSION_TO_PARENT */

char comm[TASK_COMM_LEN]; /* executable name excluding path
- access with [gs]et_task_comm (which lock
it with task_lock())
- initialized normally by setup_new_exec */
/* file system info */
int link_count, total_link_count;
#ifdef CONFIG_SYSVIPC
/* ipc stuff */
struct sysv_sem sysvsem;
#endif
#ifdef CONFIG_DETECT_HUNG_TASK
/* hung task detection */
unsigned long last_switch_count;
#endif
/* CPU-specific state of this task */
struct thread_struct thread;
/* filesystem information */
struct fs_struct *fs;
/* open file information */
struct files_struct *files;
/* namespaces */
struct nsproxy *nsproxy;
/* signal handlers */
struct signal_struct *signal;
struct sighand_struct *sighand;

sigset_t blocked, real_blocked;
sigset_t saved_sigmask;	/* restored if set_restore_sigmask() was used */
struct sigpending pending;

unsigned long sas_ss_sp;
size_t sas_ss_size;
int (*notifier)(void *priv);
void *notifier_data;
sigset_t *notifier_mask;
struct audit_context *audit_context;
#ifdef CONFIG_AUDITSYSCALL
uid_t loginuid;
unsigned int sessionid;
#endif
seccomp_t seccomp;

/* Thread group tracking */
u32 parent_exec_id;
u32 self_exec_id;
/* Protection of (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed,
* mempolicy */
spinlock_t alloc_lock;

#ifdef CONFIG_GENERIC_HARDIRQS
/* IRQ handler threads */
struct irqaction *irqaction;
#endif

/* Protection of the PI data structures: */
raw_spinlock_t pi_lock;

#ifdef CONFIG_RT_MUTEXES
/* PI waiters blocked on a rt_mutex held by this task */
struct plist_head pi_waiters;
/* Deadlock detection and priority inheritance handling */
struct rt_mutex_waiter *pi_blocked_on;
#endif

#ifdef CONFIG_DEBUG_MUTEXES
/* mutex deadlock detection */
struct mutex_waiter *blocked_on;
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
unsigned int irq_events;
unsigned long hardirq_enable_ip;
unsigned long hardirq_disable_ip;
unsigned int hardirq_enable_event;
unsigned int hardirq_disable_event;
int hardirqs_enabled;
int hardirq_context;
unsigned long softirq_disable_ip;
unsigned long softirq_enable_ip;
unsigned int softirq_disable_event;
unsigned int softirq_enable_event;
int softirqs_enabled;
int softirq_context;
#endif
#ifdef CONFIG_LOCKDEP
# define MAX_LOCK_DEPTH 48UL
u64 curr_chain_key;
int lockdep_depth;
unsigned int lockdep_recursion;
struct held_lock held_locks[MAX_LOCK_DEPTH];
gfp_t lockdep_reclaim_gfp;
#endif

/* journalling filesystem info */
void *journal_info;

/* stacked block device info */
struct bio_list *bio_list;

/* VM state */
struct reclaim_state *reclaim_state;

struct backing_dev_info *backing_dev_info;

struct io_context *io_context;

unsigned long ptrace_message;
siginfo_t *last_siginfo; /* For ptrace use.  */
struct task_io_accounting ioac;
#if defined(CONFIG_TASK_XACCT)
u64 acct_rss_mem1;	/* accumulated rss usage */
u64 acct_vm_mem1;	/* accumulated virtual memory usage */
cputime_t acct_timexpd;	/* stime + utime since last update */
#endif
#ifdef CONFIG_CPUSETS
nodemask_t mems_allowed;	/* Protected by alloc_lock */
int cpuset_mem_spread_rotor;
#endif
#ifdef CONFIG_CGROUPS
/* Control Group info protected by css_set_lock */
struct css_set *cgroups;
/* cg_list protected by css_set_lock and tsk->alloc_lock */
struct list_head cg_list;
#endif
#ifdef CONFIG_FUTEX
struct robust_list_head __user *robust_list;
#ifdef CONFIG_COMPAT
struct compat_robust_list_head __user *compat_robust_list;
#endif
struct list_head pi_state_list;
struct futex_pi_state *pi_state_cache;
#endif
#ifdef CONFIG_PERF_EVENTS
struct perf_event_context *perf_event_ctxp;
struct mutex perf_event_mutex;
struct list_head perf_event_list;
#endif
#ifdef CONFIG_NUMA
struct mempolicy *mempolicy;	/* Protected by alloc_lock */
short il_next;
#endif
atomic_t fs_excl;	/* holding fs exclusive resources */
struct rcu_head rcu;

/*
* cache last used pipe for splice
*/
struct pipe_inode_info *splice_pipe;
#ifdef	CONFIG_TASK_DELAY_ACCT
struct task_delay_info *delays;
#endif
#ifdef CONFIG_FAULT_INJECTION
int make_it_fail;
#endif
struct prop_local_single dirties;
#ifdef CONFIG_LATENCYTOP
int latency_record_count;
struct latency_record latency_record[LT_SAVECOUNT];
#endif
/*
* time slack values; these are used to round up poll() and
* select() etc timeout values. These are in nanoseconds.
*/
unsigned long timer_slack_ns;
unsigned long default_timer_slack_ns;

struct list_head	*scm_work_list;
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
/* Index of current stored address in ret_stack */
int curr_ret_stack;
/* Stack of return addresses for return function tracing */
struct ftrace_ret_stack	*ret_stack;
/* time stamp for last schedule */
unsigned long long ftrace_timestamp;
/*
* Number of functions that haven't been traced
* because of depth overrun.
*/
atomic_t trace_overrun;
/* Pause for the tracing */
atomic_t tracing_graph_pause;
#endif
#ifdef CONFIG_TRACING
/* state flags for use by tracers */
unsigned long trace;
/* bitmask of trace recursion */
unsigned long trace_recursion;
#endif /* CONFIG_TRACING */
#ifdef CONFIG_CGROUP_MEM_RES_CTLR /* memcg uses this to do batch job */
struct memcg_batch_info {
int do_batch;	/* incremented when batch uncharge started */
struct mem_cgroup *memcg; /* target memcg of uncharge */
unsigned long bytes; 		/* uncharged usage */
unsigned long memsw_bytes; /* uncharged mem+swap usage */
} memcg_batch;
#endif
};