Linux缓存写回机制
Linux缓存写回机制
在做进程安全监控的时候，拍脑袋决定的，如果发现一个进程在D状态时，即TASK_UNINTERRUPTIBLE（不可中断的睡眠状态），时间超过了8min，就将系统panic掉。恰好DB组做日志时，将整个log缓存到内存中，最后刷磁盘，结果系统就D状态了很长时间，自然panic了，中间涉及到Linux的缓存写回刷磁盘的一些机制和调优方法，写一下总结。
目前机制需要将脏页刷回到磁盘一般是以下情况：
- 脏页缓存占用的内存太多，内存空间不足；
- 脏页已经更改了很长时间，时间上已经到了临界值，需要及时刷新保持内存和磁盘上数据一致性；
- 外界命令强制刷新脏页到磁盘
- write写磁盘时检查状态刷新
内核使用pdflush线程刷新脏页到磁盘，pdflush线程个数在2和8之间，可以通过/proc/sys/vm/nr_pdflush_threads文件直接查看，具体策略机制参看源码函数__pdflush。
一、内核其他模块强制刷新
先说一下第一种和第三种情况：当内存空间不足或外界强制刷新的时候，脏页的刷新是通过调用wakeup_pdflush函数实现的，调用其函数的有do_sync、free_more_memory、try_to_free_pages。wakeup_pdflush的功能是通过background_writeout的函数实现的：
static void background_writeout(unsigned long _min_pages)
{
long min_pages = _min_pages;
struct writeback_control wbc = {
.bdi = NULL,
.sync_mode = WB_SYNC_NONE,
.older_than_this = NULL,
.nr_to_write = 0,
.nonblocking = 1,
};
for ( ; ; ) {
struct writeback_state wbs;
long background_thresh;
long dirty_thresh;
get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, NULL);
if (wbs.nr_dirty + wbs.nr_unstable < background_thresh
&& min_pages <= 0)
break;
wbc.encountered_congestion = 0;
wbc.nr_to_write = MAX_WRITEBACK_PAGES;
wbc.pages_skipped = 0;
writeback_inodes(&wbc);
min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
/* Wrote less than expected */
blk_congestion_wait(WRITE, HZ/10);
if (!wbc.encountered_congestion)
break;
}
}
}background_writeout进到一个死循环里面，通过get_dirty_limits获取脏页开始刷新的临界值background_thresh，即为dirty_background_ratio的总内存页数百分比，可以通过proc接口/proc/sys/vm/dirty_background_ratio调整，一般默认为10。当脏页超过临界值时，调用writeback_inodes写MAX_WRITEBACK_PAGES（1024）个页，直到脏页比例低于临界值。
二、内核定时器启动刷新
内核在启动的时候在page_writeback_init初始化wb_timer定时器，超时时间是dirty_writeback_centisecs，单位是0.01秒，可以通过/proc/sys/vm/dirty_writeback_centisecs调节。wb_timer的触发函数是wb_timer_fn，最终是通过wb_kupdate实现。
static void wb_kupdate(unsigned long arg)
{
sync_supers();
get_writeback_state(&wbs);
oldest_jif = jiffies - (dirty_expire_centisecs * HZ) / 100;
start_jif = jiffies;
next_jif = start_jif + (dirty_writeback_centisecs * HZ) / 100;
nr_to_write = wbs.nr_dirty + wbs.nr_unstable +
(inodes_stat.nr_inodes - inodes_stat.nr_unused);
while (nr_to_write > 0) {
wbc.encountered_congestion = 0;
wbc.nr_to_write = MAX_WRITEBACK_PAGES;
writeback_inodes(&wbc);
if (wbc.nr_to_write > 0) {
if (wbc.encountered_congestion)
blk_congestion_wait(WRITE, HZ/10);
else
break; /* All the old data is written */
}
nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
}
if (time_before(next_jif, jiffies + HZ))
next_jif = jiffies + HZ;
if (dirty_writeback_centisecs)
mod_timer(&wb_timer, next_jif);
}上面的代码没有拷贝全。内核首先将超级块信息刷新到文件系统上，然后获取oldest_jif作为wbc的参数只刷新已修改时间大于dirty_expire_centisecs的脏页，dirty_expire_centisecs参数可以通过/proc/sys/vm/dirty_expire_centisecs调整。
三、WRITE写文件刷新缓存
用户态使用WRITE函数写文件时也有可能要刷新脏页，generic_file_buffered_write函数会在将写的内存页标记为脏之后，根据条件刷新磁盘以平衡当前脏页比率，参看balance_dirty_pages_ratelimited函数：
void balance_dirty_pages_ratelimited(struct address_space *mapping)
{
static DEFINE_PER_CPU(int, ratelimits) = 0;
long ratelimit;
ratelimit = ratelimit_pages;
if (dirty_exceeded)
ratelimit = 8;
/*
* Check the rate limiting. Also, we do not want to throttle real-time
* tasks in balance_dirty_pages(). Period.
*/
if (get_cpu_var(ratelimits)++ >= ratelimit) {
__get_cpu_var(ratelimits) = 0;
put_cpu_var(ratelimits);
balance_dirty_pages(mapping);
return;
}
put_cpu_var(ratelimits);
}balance_dirty_pages_ratelimited函数通过ratelimit_pages调节刷新（调用balance_dirty_pages函数）的次数，每ratelimit_pages次调用才会刷新一次，具体刷新过程看balance_dirty_pages函数：
static void balance_dirty_pages(struct address_space *mapping)
{
struct writeback_state wbs;
long nr_reclaimable;
long background_thresh;
long dirty_thresh;
unsigned long pages_written = 0;
unsigned long write_chunk = sync_writeback_pages();
struct backing_dev_info *bdi = mapping->backing_dev_info;
for (;;) {
struct writeback_control wbc = {
.bdi = bdi,
.sync_mode = WB_SYNC_NONE,
.older_than_this = NULL,
.nr_to_write = write_chunk,
};
get_dirty_limits(&wbs, &background_thresh,
&dirty_thresh, mapping);
nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable;
if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
break;
if (!dirty_exceeded)
dirty_exceeded = 1;
/* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
* Unstable writes are a feature of certain networked
* filesystems (i.e. NFS) in which data may have been
* written to the server's write cache, but has not yet
* been flushed to permanent storage.
*/
if (nr_reclaimable) {
writeback_inodes(&wbc);
get_dirty_limits(&wbs, &background_thresh,
&dirty_thresh, mapping);
nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable;
if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
break;
pages_written += write_chunk - wbc.nr_to_write;
if (pages_written >= write_chunk)
break; /* We've done our duty */
}
blk_congestion_wait(WRITE, HZ/10);
}
if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh && dirty_exceeded)
dirty_exceeded = 0;
if (writeback_in_progress(bdi))
return; /* pdflush is already working this queue */
/*
* In laptop mode, we wait until hitting the higher threshold before
* starting background writeout, and then write out all the way down
* to the lower threshold. So slow writers cause minimal disk activity.
*
* In normal mode, we start background writeout at the lower
* background_thresh, to keep the amount of dirty memory low.
*/
if ((laptop_mode && pages_written) ||
(!laptop_mode && (nr_reclaimable > background_thresh)))
pdflush_operation(background_writeout, 0);
}函数走进一个死循环，通过get_dirty_limits获取dirty_background_ratio和dirty_ratio对应的内存页数值，当24行做判断，如果脏页大于dirty_thresh，则调用writeback_inodes开始刷缓存到磁盘，如果一次没有将脏页比率刷到dirty_ratio之下，则用blk_congestion_wait阻塞写，然后反复循环，直到比率降低到dirty_ratio；当比率低于dirty_ratio之后，但脏页比率大于dirty_background_ratio，则用pdflush_operation启用background_writeout，pdflush_operation是非阻塞函数，唤醒pdflush后直接返回，background_writeout在有pdflush调用。
如此可知：WRITE写的时候，缓存超过dirty_ratio，则会阻塞写操作，回刷脏页，直到缓存低于dirty_ratio；如果缓存高于background_writeout，则会在写操作时，唤醒pdflush进程刷脏页，不阻塞写操作。
四，问题总结
导致进程D状态大部分是因为第3种和第4种情况：有大量写操作，缓存由Linux系统管理，一旦脏页累计到一定程度，无论是继续写还是fsync刷新，都会使进程D住。