一、需求

通过阿里云启动项目时，使用Vuepress build编译静态页面时内存需要800MB，导致内存不够，因此考虑使用swap方式，置换一些内存资源存放swap磁盘。

[root@xxx myblog]# npm run docs:dev> myblog@1.0.0 docs:dev
> vuepress dev docswait Extracting site metadata...
tip Apply theme @vuepress/theme-default ...
warning Invalid value for "plugin": expected a String, Function or Object but got Array.
warning An error was encountered in plugin "@vuepress/back-to-top"
tip Apply plugin container (i.e. "vuepress-plugin-container") ...
tip Apply plugin @vuepress/register-components (i.e. "@vuepress/plugin-register-components") ...
tip Apply plugin @vuepress/active-header-links (i.e. "@vuepress/plugin-active-header-links") ...
tip Apply plugin @vuepress/search (i.e. "@vuepress/plugin-search") ...
tip Apply plugin @vuepress/nprogress (i.e. "@vuepress/plugin-nprogress") ...
tip Apply plugin copyright (i.e. "vuepress-plugin-copyright") ...
tip Apply plugin sitemap (i.e. "vuepress-plugin-sitemap") ...
tip Apply plugin baidu-autopush (i.e. "vuepress-plugin-baidu-autopush") ...
tip Apply plugin @vuepress/medium-zoom (i.e. "@vuepress/plugin-medium-zoom") ...
tip Apply plugin img-lazy (i.e. "vuepress-plugin-img-lazy") ...
tip Apply plugin @vssue/vssue (i.e. "@vssue/vuepress-plugin-vssue") ...
tip Apply plugin one-click-copy (i.e. "vuepress-plugin-one-click-copy") ...● Client █████████████████████████ building (68%) 2689/2748 modules 59 activeurl-loader › docs/blogs/interview/image/java/link.pngℹ ｢wds｣: Project is running at http://0.0.0.0:8080/
ℹ ｢wds｣: webpack output is served from /
ℹ ｢wds｣: Content not from webpack is served from /root/project/myblog/docs/.vuepress/public
ℹ ｢wds｣: 404s will fallback to /index.html
Language does not exist: c++
Language does not exist: c++
Language does not exist: c++
Language does not exist: c++
Killed

swap分区是Linux操作系统中的一种虚拟化内存技术，将硬盘空间作为内存使用。由于内存和磁盘的读写性能差异较大，Linux会在内存充裕时将空闲内存用于缓存磁盘数据，以提高I/O性能。相对的在内存紧张时Linux会将这些缓存回收，将脏页回写到磁盘中。而在进程的地址空间中，如heap，stack等匿名页，在磁盘上并没有对应的文件，但同样有回收到磁盘上以释放出空闲内存的需求。swap机制通过在磁盘上开辟专用的swap分区作为匿名页的backing storage，满足了这一需求。

在Linux上可以使用swapon -s命令查看当前系统上正在使用的交换空间有哪些，以及相关信息：

[root@xxx myblog]# swapon
NAME      TYPE SIZE   USED PRIO
/etc/swap file   2G 677.9M   -2

二、SWAP 创建

Linux支持两种形式的swap分区： 使用分区空间swap disk和使用分区文件swap file。前者是一个专用于做swap的块设备，作为裸设备提供给swap机制操作；后者则是存放在文件系统上的一个特定文件，其实现依赖于不同的文件系统，会有所区别。

分区文件swap file

【1】创建swap文件

[root@xxx myblog]# fallocate -l 2G /etc/swap #指定文件为2G

【2】设置该文件为swap文件

[root@xxx myblog]# mkswap /etc/swap
Setting up swapspace version 1, size = 2097148 KiB
no label, UUID=5b9e4232-dad5-4dbd-9805-f2296452e6f8

【3】启动swap文件

[root@xxx myblog]# swapon /etc/swap
swapon: /etc/swap: insecure permissions 0644, 0600 suggested.

【4】使swap文件永久生效

vim /etc/fstab

【5】在fstab末尾添加如下内容

/etc/swap  swap   swap  defaults  0 0

【6】更改swap配置

vim /etc/sysctl.conf

【7】添加如下内容：值越大表示越倾向于使用swap空间

vm.swappiness=30

【8】重启生效

init 6

分区空间swap disk

【1】创建分区：并设置为swap格式

fdisk /dev/sdb

参数	说明
n	创建分区
p	创建主分区
1	创建分区1
两次回车	起始扇区和Last扇区选择默认
t	转换分区格式
82	转换为swap空间
p	查看已创建的分区结果
w	保存退出
【2】格式化为swap空间

mkswap /dev/sdb1

【3】启用swap

swapon /dev/sdb1

【4】编辑配置文件，设为开机自动挂载

vim /etc/fstab

【5】fstab中添加如下内容：

/dev/sdb1  swap   swap  defaults  0 0

【6】设置自动启用所有swap空间

swapon -a

【7】重启验证

init 6

可通过swapon和swapoff命令开启或关闭对应的swap分区。通过cat /proc/swaps或swapon -s可以查看使用中的swap分区的状态。

[root@xxx myblog]# swapon -s
Filename                                Type            Size    Used    Priority
/etc/swap                               file    2097148 678772  -2

移除交换(Swap)文件

通过以下命令来移除交换Swap文件，或者通过命令删除/etc/fstab中的交换文件

[root@xxx]# sudo swapoff -v /swapfile

三、整Swappiness值

Swappiness是Linux内核的一个属性，用于定义交换空间的使用频率。如您所知，RAM比硬盘驱动器快。因此，每次您需要使用交换时，您都会注意到某些进程和应用程序运行速度会变慢。但是，您可以调整系统以使用比交换更多的RAM。这有助于提高整体系统性能。通常，默认的swappiness值为30。此值越小，将使用的RAM越多。

要验证swappiness值，请运行以下命令：

[root@xxxx ~]# cat /proc/sys/vm/swappiness
30

如果想要修改swappiness的值，可以编辑/etc/sysctl.conf文件。并添加以下以下内容。

vm.swappiness=20

为了应用更改，则需要重新启动系统。这样Linux内核将使用更多的RAM和更少的交换，但是当你的RAM内存严重满时它仍然会交换。通常，当您的RAM超过4Gb时，建议使用此设置。

四、页面回收机制

Linux触发页面回收有三种情况：
【1】直接回收：alloc_pages()分配物理页，内存紧缺时，会陷入回收机制，同步触发；
【2】周期性回收：当系统内存触发低水位时，唤醒kswapd线程，异步回收内存；
【3】slab收割机制：当内存紧缺时，直接回收，周期性回收，都会调用slab收割机回收，不过这里是内核的内存分配；

kswapd_wait等待队列： 等待队列用于使进程等待某一事件发生，而无需频繁轮询，进程在等待期间睡眠。在某事件发生时，由内核自动唤醒。

setup_arch()-->paging_init()-->bootmem_init()->zone_sizes_init()-->free_area_init_node()-->free_area_init_core()

kswapd_wait等待队列在free_area_init_core中进行初始化，每个内存节点一个。kswapd内核线程在kswapd_wait等待队列上等待TASK_INTERRUPTIBLE事件发生。

static void __paginginit free_area_init_core(struct pglist_data *pgdat,unsigned long node_start_pfn, unsigned long node_end_pfn,unsigned long *zones_size, unsigned long *zholes_size)
{
...init_waitqueue_head(&pgdat->kswapd_wait);init_waitqueue_head(&pgdat->pfmemalloc_wait);pgdat_page_ext_init(pgdat);...
}

kswapd内核线程： kswapd内核线程负责在内存不足的情况下进行页面回收，为每NUMA内存节点创建一个kswap%d的内核线程。其中kswapd函数是内核线程kswapd的入口。

/** 一个pglist_data，对应一个内存节点，是最顶层的内存管理数据结构* 主要包括三部分：* 1.描述zone* 2.描述内存节点的信息；* 3.和页面回收相关；*/
typedef struct pglist_data {int node_id;wait_queue_head_t kswapd_wait;struct task_struct *kswapd; /* Protected bymem_hotplug_begin/end() */int kswapd_order;enum zone_type kswapd_highest_zoneidx;struct lruvec		__lruvec;  ///lru链表向量(包括所有，5种lru链表)} pg_data_t;

wakeup_kswapd唤醒kswaped内核线程： 分配内存路径上的唤醒函数wakeup_kswapd把kswapd_order和kswapd_highest_zoneidx作为参数传递给kswaped内核线程；

alloc_page()->__alloc_pages_nodemask()->__alloc_pages_slowpth()->wake_all_kswapds()->wakeup_kswapd()void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,enum zone_type highest_zoneidx)
{pg_data_t *pgdat;enum zone_type curr_idx;if (!managed_zone(zone))return;if (!cpuset_zone_allowed(zone, gfp_flags))return;pgdat = zone->zone_pgdat;///准备本内存节点的kswapd_order和kswapd_highest_zoneidxcurr_idx = READ_ONCE(pgdat->kswapd_highest_zoneidx);if (curr_idx == MAX_NR_ZONES || curr_idx < highest_zoneidx)WRITE_ONCE(pgdat->kswapd_highest_zoneidx, highest_zoneidx);if (READ_ONCE(pgdat->kswapd_order) < order)WRITE_ONCE(pgdat->kswapd_order, order);if (!waitqueue_active(&pgdat->kswapd_wait))return;/* Hopeless node, leave it to direct reclaim if possible */if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ||(pgdat_balanced(pgdat, order, highest_zoneidx) &&!pgdat_watermark_boosted(pgdat, highest_zoneidx))) {/** There may be plenty of free memory available, but it's too* fragmented for high-order allocations.  Wake up kcompactd* and rely on compaction_suitable() to determine if it's* needed.	If it fails, it will defer subsequent attempts to* ratelimit its work.*/if (!(gfp_flags & __GFP_DIRECT_RECLAIM))wakeup_kcompactd(pgdat, order, highest_zoneidx);return;}trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, highest_zoneidx, order,gfp_flags);///唤醒kswapd_wait队列wake_up_interruptible(&pgdat->kswapd_wait);
}

回收函数kswapd

static int kswapd(void *p)
{...///PF_MEMALLOC允许使用系统预留内存，即不考虑水位tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;for ( ; ; ) {bool ret;///回收页面数量，2的order次幂alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);///classzone_idx内核线程扫描和回收的最高zonehighest_zoneidx = kswapd_highest_zoneidx(pgdat,highest_zoneidx);kswapd_try_sleep:///睡眠，等待wakeup_kswapd唤醒kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,highest_zoneidx);
...reclaim_order = balance_pgdat(pgdat, alloc_order,highest_zoneidx);if (reclaim_order < alloc_order)goto kswapd_try_sleep;}tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);return 0;
}

kswapd内核线程扫描过程： kswapd()->balance_pgdat()

/****************************************************************************** 回收页面的主函数:** highmem->normal->dma, 从高端往低端方向,查找处于不平衡状态，* 即free_pages <= high_wmark_pagesend_zone的zone* * ****************************************************************************/
static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
{///用于内存碎片化unsigned long nr_boost_reclaim;
...nr_boost_reclaim = 0;for (i = 0; i <= highest_zoneidx; i++) {zone = pgdat->node_zones + i;if (!managed_zone(zone))continue;nr_boost_reclaim += zone->watermark_boost;zone_boosts[i] = zone->watermark_boost;}boosted = nr_boost_reclaim;restart:sc.priority = DEF_PRIORITY;do {...///检查这个节点中是否有合格的zone，其水位高于高水位且能分配2的sc.order次幂个连续的物理页面balanced = pgdat_balanced(pgdat, sc.order, highest_zoneidx);///若所有zone都不合格，关闭nr_boost_reclaim，重新检查一次if (!balanced && nr_boost_reclaim) {nr_boost_reclaim = 0;goto restart;}//若符合条件，不需要回收，直接跳出if (!nr_boost_reclaim && balanced)goto out;...///老化匿名页面的活跃链表age_active_anon(pgdat, &sc);...///真正扫描和页回收函数，扫描的参数和结果存放在struct scan_control中，///返回true表明回收了所需要的页面，不需要再提高扫描优先级if (kswapd_shrink_node(pgdat, &sc))raise_priority = false;...///加大扫描粒度if (raise_priority || !nr_reclaimed)sc.priority--;} while (sc.priority >= 1);...out:/* If reclaim was boosted, account for the reclaim done in this pass *////若设置了nr_boost_reclaim，唤醒kcompacted线程if (boosted) {...wakeup_kcompactd(pgdat, pageblock_order, highest_zoneidx);}...return sc.order;
}

对活跃链表中页面的老化：kswapd()->balance_pgdat()->age_active_anon()

///老化匿名页面的活跃链表
static void age_active_anon(struct pglist_data *pgdat,struct scan_control *sc)
{struct mem_cgroup *memcg;struct lruvec *lruvec;if (!total_swap_pages)return;lruvec = mem_cgroup_lruvec(NULL, pgdat);if (!inactive_is_low(lruvec, LRU_INACTIVE_ANON))return;memcg = mem_cgroup_iter(NULL, NULL, NULL);do {lruvec = mem_cgroup_lruvec(memcg, pgdat);shrink_active_list(SWAP_CLUSTER_MAX, lruvec,sc, LRU_ACTIVE_ANON);memcg = mem_cgroup_iter(NULL, memcg, NULL);} while (memcg);
}

执行回收：kswapd()->balance_pgdat()->kswapd_shrink_node()->shrink_node()->shrink_node_memcgs()

static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
{struct mem_cgroup *target_memcg = sc->target_mem_cgroup;struct mem_cgroup *memcg;memcg = mem_cgroup_iter(target_memcg, NULL, NULL);do {///获取LRU链表的集合struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);unsigned long reclaimed;unsigned long scanned;/** This loop can become CPU-bound when target memcgs* aren't eligible for reclaim - either because they* don't have any reclaimable pages, or because their* memory is explicitly protected. Avoid soft lockups.*/cond_resched();mem_cgroup_calculate_protection(target_memcg, memcg);if (mem_cgroup_below_min(memcg)) {/** Hard protection.* If there is no reclaimable memory, OOM.*/continue;} else if (mem_cgroup_below_low(memcg)) {/** Soft protection.* Respect the protection only as long as* there is an unprotected supply* of reclaimable memory from other cgroups.*/if (!sc->memcg_low_reclaim) {sc->memcg_low_skipped = 1;continue;}memcg_memory_event(memcg, MEMCG_LOW);}reclaimed = sc->nr_reclaimed;scanned = sc->nr_scanned;///扫描回收lru链表shrink_lruvec(lruvec, sc);///扫描回收slab链表shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,sc->priority);/* Record the group's reclaim efficiency */vmpressure(sc->gfp_mask, memcg, false,sc->nr_scanned - scanned,sc->nr_reclaimed - reclaimed);} while ((memcg = mem_cgroup_iter(target_memcg, memcg, NULL)));
}

回收函数shrink_lruvec()

static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
{unsigned long nr[NR_LRU_LISTS];unsigned long targets[NR_LRU_LISTS];unsigned long nr_to_scan;enum lru_list lru;unsigned long nr_reclaimed = 0;unsigned long nr_to_reclaim = sc->nr_to_reclaim;struct blk_plug plug;bool scan_adjusted;///计算每个链表应该扫描的页面数量，结果放在nr[]get_scan_count(lruvec, sc, nr);///全局回收，优化当内存紧缺时，触发直接回收scan_adjusted = (!cgroup_reclaim(sc) && !current_is_kswapd() &&sc->priority == DEF_PRIORITY);///遍历所有链表，回收页面///主要处理不活跃匿名页面，活跃文件映射页面和不活跃文件映射页面while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||nr[LRU_INACTIVE_FILE]) {unsigned long nr_anon, nr_file, percentage;unsigned long nr_scanned;for_each_evictable_lru(lru) {if (nr[lru]) {nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);nr[lru] -= nr_to_scan;//扫描链表，回收页面，返回成功回收的页面数量nr_reclaimed += shrink_list(lru, nr_to_scan,lruvec, sc);}}cond_resched();///没完成回收任务，或设置了scan_adjusted，继续进行页面扫描if (nr_reclaimed < nr_to_reclaim || scan_adjusted)continue;...scan_adjusted = true;}blk_finish_plug(&plug);sc->nr_reclaimed += nr_reclaimed;///老化活跃链表///如果不活跃链表页面数量太少，从活跃链表迁移一部分页面到不活跃链表if (total_swap_pages && inactive_is_low(lruvec, LRU_INACTIVE_ANON))shrink_active_list(SWAP_CLUSTER_MAX, lruvec,sc, LRU_ACTIVE_ANON);
}

shrink_lruvec()->shrink_list()

static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,struct lruvec *lruvec, struct scan_control *sc){if (is_active_lru(lru)) {///扫描活跃的文件映射链表if (sc->may_deactivate & (1 << is_file_lru(lru)))shrink_active_list(nr_to_scan, lruvec, sc, lru);elsesc->skipped_deactivate = 1;return 0;}///扫描不活跃链表return shrink_inactive_list(nr_to_scan, lruvec, sc, lru);}

扫描活跃链表函数shrink_active_list()实现:

/************************************************************************************** func:扫描活跃链表，包括匿名页或文件映射页面，*      把最近没访问的页面，从活跃链表尾部移到不活跃链表头部* nr_to_scan: 待扫描页面的数量* lruvec：LRU链表集合* sc：页面扫描控制参数
*  lru： 待扫描的LRU链表类型
*************************************************************************************/
static void shrink_active_list(unsigned long nr_to_scan,struct lruvec *lruvec,struct scan_control *sc,enum lru_list lru)
{unsigned long nr_taken;unsigned long nr_scanned;unsigned long vm_flags;///定义三个临时链表LIST_HEAD(l_hold);	/* The pages which were snipped off */LIST_HEAD(l_active);LIST_HEAD(l_inactive);struct page *page;unsigned nr_deactivate, nr_activate;unsigned nr_rotated = 0;///判断是否为文件映射链表int file = is_file_lru(lru);///获取内存节点struct pglist_data *pgdat = lruvec_pgdat(lruvec);lru_add_drain();spin_lock_irq(&lruvec->lru_lock);///将页面批量迁移到临时链表l_hold中nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,&nr_scanned, sc, lru);///增加内存节点NR_ISOLATED_ANON计数__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);if (!cgroup_reclaim(sc))__count_vm_events(PGREFILL, nr_scanned);__count_memcg_events(lruvec_memcg(lruvec), PGREFILL, nr_scanned);spin_unlock_irq(&lruvec->lru_lock);///扫描临时链表l_hold，有些页面放到不活跃链表，有些会放回到活跃链表while (!list_empty(&l_hold)) {cond_resched();page = lru_to_page(&l_hold);list_del(&page->lru);///如果不能回收，放入不能回收链表if (unlikely(!page_evictable(page))) {putback_lru_page(page);continue;}if (unlikely(buffer_heads_over_limit)) {if (page_has_private(page) && trylock_page(page)) {if (page_has_private(page))try_to_release_page(page, 0);unlock_page(page);}}///page_referenced()返回该页面最近访问，应用pte个数，若返回0，表示最近没访问if (page_referenced(page, 0, sc->target_mem_cgroup,&vm_flags)) {/** Identify referenced, file-backed active pages and* give them one more trip around the active list. So* that executable code get better chances to stay in* memory under moderate memory pressure.  Anon pages* are not likely to be evicted by use-once streaming* IO, plus JVM can create lots of anon VM_EXEC pages,* so we ignore them here.*/if ((vm_flags & VM_EXEC) && page_is_file_lru(page)) {nr_rotated += thp_nr_pages(page);///放回活跃链表list_add(&page->lru, &l_active); continue;}}ClearPageActive(page);	/* we are de-activating */SetPageWorkingset(page);///加入不活跃链表list_add(&page->lru, &l_inactive);}/** Move pages back to the lru list.*/spin_lock_irq(&lruvec->lru_lock);///将l_active,l_inactive分别加入到相应的链表nr_activate = move_pages_to_lru(lruvec, &l_active);nr_deactivate = move_pages_to_lru(lruvec, &l_inactive);/* Keep all free pages in l_active list */list_splice(&l_inactive, &l_active);__count_vm_events(PGDEACTIVATE, nr_deactivate);__count_memcg_events(lruvec_memcg(lruvec), PGDEACTIVATE, nr_deactivate);__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);spin_unlock_irq(&lruvec->lru_lock);mem_cgroup_uncharge_list(&l_active);free_unref_page_list(&l_active);trace_mm_vmscan_lru_shrink_active(pgdat->node_id, nr_taken, nr_activate,nr_deactivate, nr_rotated, sc->priority, file);
}

扫描不活跃链表shrink_inactive_list()实现:

///扫描不活跃LRU链表，尝试回收页面，返回已经回收的页面数量
static noinline_for_stack unsigned long
shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,struct scan_control *sc, enum lru_list lru)
{LIST_HEAD(page_list);unsigned long nr_scanned;unsigned int nr_reclaimed = 0;unsigned long nr_taken;struct reclaim_stat stat;bool file = is_file_lru(lru);enum vm_event_item item;struct pglist_data *pgdat = lruvec_pgdat(lruvec);bool stalled = false;while (unlikely(too_many_isolated(pgdat, file, sc))) {if (stalled)return 0;/* wait a bit for the reclaimer. *////太多进程在直接回收页面，睡眠，避免内存抖动msleep(100);  stalled = true;/* We are about to die and free our memory. Return now. */if (fatal_signal_pending(current))return SWAP_CLUSTER_MAX;}lru_add_drain();spin_lock_irq(&lruvec->lru_lock);///分离页面到临时页表nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &page_list,&nr_scanned, sc, lru);///增加内存节点NR_ISOLATED_ANON计数__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);item = current_is_kswapd() ? PGSCAN_KSWAPD : PGSCAN_DIRECT;if (!cgroup_reclaim(sc))__count_vm_events(item, nr_scanned);__count_memcg_events(lruvec_memcg(lruvec), item, nr_scanned);__count_vm_events(PGSCAN_ANON + file, nr_scanned);spin_unlock_irq(&lruvec->lru_lock);if (nr_taken == 0)return 0;///执行回收页面，返回nr_reclaimed个nr_reclaimed = shrink_page_list(&page_list, pgdat, sc, &stat, false);spin_lock_irq(&lruvec->lru_lock);///page_list链表剩余页面迁回不活跃链表move_pages_to_lru(lruvec, &page_list);///减少NR_ISOLATED_ANON计数__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);item = current_is_kswapd() ? PGSTEAL_KSWAPD : PGSTEAL_DIRECT;if (!cgroup_reclaim(sc))__count_vm_events(item, nr_reclaimed);__count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);__count_vm_events(PGSTEAL_ANON + file, nr_reclaimed);spin_unlock_irq(&lruvec->lru_lock);lru_note_cost(lruvec, file, stat.nr_pageout);mem_cgroup_uncharge_list(&page_list);free_unref_page_list(&page_list);/** If dirty pages are scanned that are not queued for IO, it* implies that flushers are not doing their job. This can* happen when memory pressure pushes dirty pages to the end of* the LRU before the dirty limits are breached and the dirty* data has expired. It can also happen when the proportion of* dirty pages grows not through writes but through memory* pressure reclaiming all the clean cache. And in some cases,* the flushers simply cannot keep up with the allocation* rate. Nudge the flusher threads in case they are asleep.*/if (stat.nr_unqueued_dirty == nr_taken)wakeup_flusher_threads(WB_REASON_VMSCAN);sc->nr.dirty += stat.nr_dirty;sc->nr.congested += stat.nr_congested;sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;sc->nr.writeback += stat.nr_writeback;sc->nr.immediate += stat.nr_immediate;sc->nr.taken += nr_taken;if (file)sc->nr.file_taken += nr_taken;trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,nr_scanned, nr_reclaimed, &stat, sc->priority, file);return nr_reclaimed;
}