linux进程与线程的区别

个人理解，仅供自己参考。

本文参考

http://www.cnblogs.com/blueclue/archive/2010/07/16/1778855.html

http://blog.csdn.net/forrest2009/article/details/6413756

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_CTXSWint oncpu;#endif#endifint 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_TRACEunsigned int btrace_seq;#endifunsigned int policy;cpumask_t cpus_allowed;#ifdef CONFIG_TREE_PREEMPT_RCUint 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;#endifstruct list_head tasks;struct plist_node pushable_tasks;struct mm_struct *mm, *active_mm;#if defined(SPLIT_RSS_COUNTING)struct task_rss_statrss_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_ACCOUNTINGcputime_t prev_utime, prev_stime;#endifunsigned 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_AUDITSYSCALLuid_t loginuid;unsigned int sessionid;#endifseccomp_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_IRQFLAGSunsigned 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 48ULu64 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_CPUSETSnodemask_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_FUTEXstruct robust_list_head __user *robust_list;#ifdef CONFIG_COMPATstruct compat_robust_list_head __user *compat_robust_list;#endifstruct list_head pi_state_list;struct futex_pi_state *pi_state_cache;#endif#ifdef CONFIG_PERF_EVENTSstruct perf_event_context *perf_event_ctxp;struct mutex perf_event_mutex;struct list_head perf_event_list;#endif#ifdef CONFIG_NUMAstruct mempolicy *mempolicy;/* Protected by alloc_lock */short il_next;#endifatomic_t fs_excl;/* holding fs exclusive resources */struct rcu_head rcu;/* * cache last used pipe for splice */struct pipe_inode_info *splice_pipe;#ifdefCONFIG_TASK_DELAY_ACCTstruct task_delay_info *delays;#endif#ifdef CONFIG_FAULT_INJECTIONint make_it_fail;#endifstruct prop_local_single dirties;#ifdef CONFIG_LATENCYTOPint 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};