Why IO bond process increase cpu load
Author: Tony
Date:2013-01-11
About /proc/loadavg
下面是从网上流行的解释:
前三个数字表示的是1、5、15分钟内的CPU平均处于运行状态的进程个数,后面的两个,一个的分子是正在运行的进程数,分母是进程总数;另一个是最近运行的进程PID
Copy file will
increase cpu load
如果在linux系统上运行大文件的copy操作或者大量的读写磁盘操作,系统负载会显著增大。Why?
进行磁盘操作的时候,应该是IO操作,CPU大部分时间应该是等待IO操作完成,不应该占用CPU时间的,为什么load会增大呢?难道linux内核在等待IO的时候,仍然霸占CPU,不能被其他的进程使用?(内核应该不会这么傻,呵呵)
If process can’t get data from disk immediately,what to do?
当进程等待IO数据的时候,最终会到block层的get_request_wait(或类似的函数)
看一下这个函数的实现:(/block/blk-core.c)
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static struct request *get_request_wait(struct request_queue *q, int rw_flags,
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struct bio *bio)
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{
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const bool is_sync = rw_is_sync(rw_flags) != 0;
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struct request *rq;
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rq = get_request(q, rw_flags, bio, GFP_NOIO);
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while (!rq) {
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DEFINE_WAIT(wait);
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struct io_context *ioc;
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struct request_list *rl = &q->rq;
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/*修改当前进程的状态为TASK_UNINTERRUPTIBLE*/
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prepare_to_wait_exclusive(&rl->wait[is_sync], &wait,
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TASK_UNINTERRUPTIBLE);
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trace_block_sleeprq(q, bio, rw_flags & 1);
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__generic_unplug_device(q);
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spin_unlock_irq(q->queue_lock);
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/*调用函数进行调度*/
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io_schedule();
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/*
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* After sleeping, we become a "batching" process and
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* will be able to allocate at least one request, and
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* up to a big batch of them for a small period time.
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* See ioc_batching, ioc_set_batching
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*/
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ioc = current_io_context(GFP_NOIO, q->node);
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ioc_set_batching(q, ioc);
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spin_lock_irq(q->queue_lock);
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finish_wait(&rl->wait[is_sync], &wait);
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rq = get_request(q, rw_flags, bio, GFP_NOIO);
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};
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return rq;
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}
函数调用了io_schedule(有些会调用io_schedule_timeout,这个函数的不同是,至少sleep多长时间)。我们看一下io_schedule的代码:(kernel/sched.c)
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/*
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* This task is about to go to sleep on IO. Increment rq->nr_iowait so
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* that process accounting knows that this is a task in IO wait state.
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*/
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void __sched io_schedule(void)
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{
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struct rq *rq = raw_rq();
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delayacct_blkio_start();
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atomic_inc(&rq->nr_iowait);
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current->in_iowait = 1;
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/*调用schedule函数,让出的CPU*/
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schedule();
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current->in_iowait = 0;
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/*增加当前CPU的等待IO的进程数*/
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atomic_dec(&rq->nr_iowait);
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delayacct_blkio_end();
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}
从这个函数可以看出,当进程IO操作的时候,会从当前CPU的running队列移除,并让出CPU。那按照网上的解释,不应该增加load值啊。看来需要看看load的真正含义了。
How the kernel calculate load?
/proc/loadavg对应的代码是fs/proc/loadavg.c
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static int loadavg_proc_show(struct seq_file *m, void *v)
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{
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unsigned long avnrun[3];
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get_avenrun(avnrun, FIXED_1/200, 0);
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seq_printf(m, "%lu.%02lu %lu.%02lu %lu.%02lu %ld/%d %d\n",
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LOAD_INT(avnrun[0]), LOAD_FRAC(avnrun[0]),
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LOAD_INT(avnrun[1]), LOAD_FRAC(avnrun[1]),
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LOAD_INT(avnrun[2]), LOAD_FRAC(avnrun[2]),
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nr_running(), nr_threads,
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task_active_pid_ns(current)->last_pid);
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return 0;
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}
这里通过get_avenrun函数获取(kernel/sched.c)
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void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
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{
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loads[0] = (avenrun[0] + offset) << shift;
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loads[1] = (avenrun[1] + offset) << shift;
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loads[2] = (avenrun[2] + offset) << shift;
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}
这里实际读取的全局变量avenrun。Avenrun的更新在函数calc_global_load
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void calc_global_load(void)
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{
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unsigned long upd = calc_load_update + 10;
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long active;
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if (time_before(jiffies, upd))
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return;
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active = atomic_long_read(&calc_load_tasks);
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active = active > 0 ? active * FIXED_1 : 0;
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avenrun[0] = calc_load(avenrun[0], EXP_1, active);
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avenrun[1] = calc_load(avenrun[1], EXP_5, active);
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avenrun[2] = calc_load(avenrun[2], EXP_15, active);
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calc_load_update += LOAD_FREQ;
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}
这里实际读取的calc_load_tasks变量,这个变量在函数calc_load_account_active中被更新。(这个函数可能在每个tick的时候通过update_cpu_load调用,也可能在cpu选择idle进程的时候调用)
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static void calc_load_account_active(struct rq *this_rq)
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{
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long nr_active, delta;
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nr_active = this_rq->nr_running;
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/*这里不仅包含的running的进程,也包含了uninterruptible的进程*/
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nr_active += (long) this_rq->nr_uninterruptible;
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if (nr_active != this_rq->calc_load_active) {
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delta = nr_active - this_rq->calc_load_active;
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this_rq->calc_load_active = nr_active;
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atomic_long_add(delta, &calc_load_tasks);
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}
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}
Conclusion
内核统计load的含义的是,所有cpu上的处于running状态和uninterruptible状态的进程,由于IO操作的时候,进程会进入uninterruptible状态,所有会增大系统的负载。这个时候负载虽然高,但是CPU仍然是比较空闲的,也就说IO导致的负载过高,对cpu bound型进程的影响不是太大。
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