未开启CONFIGMEMCG, CONFIGSWAP.

1. scan_control

linux-3.10.86/mm/vmscan.c

struct scan_control {

    /* Incremented by the number of inactive pages that were scanned */
    unsigned long nr_scanned;

    /* How many pages shrink_list() should reclaim 
    问题:nr_to_reclaim和nr_scanned的关系?
    答:nr_to_reclaim是个setting data, nr_reclaimed是runtime date.
    通常是先给struct scan_control sc设置好这个目标, 
    然后启动回收. 
    在 sum of shrink_list() > nr_to_reclaim后中断回收, see shrink_lruvec() or do_try_to_free_pages().
    */
    unsigned long nr_to_reclaim;

    /*
    这里的may类似may I ..., may的意思是 是否可以, 是否允许
    */
    int may_writepage;

    /*
    [Understanding the Linux Kernel, 3rd Edition]p695
    Lower priority implies scanning more pages.
    */
    int priority;

};

get_scan_count
{
        size = get_lru_size(lruvec, lru);
        scan = size >> sc->priority;
        //扫描的量 与 list的大小 成比例
}

shrink_lruvec
|--//1. 根据优先级等 给数组nr[]赋值
|--get_scan_count(lruvec, sc, nr); 

2. watermark

2.1 low watermark

发现低于low watermark时, 唤醒kswapd:

__alloc_pages_nodemask
|-- int alloc_flags = ALLOC_WMARK_LOW|ALLOC_CPUSET;
|--page = get_page_from_freelist(,alloc_flags,)
|--if (unlikely(!page)) 
|   |--__alloc_pages_slowpath()
|   |   |--if (!(gfp_mask & __GFP_NO_KSWAPD))
|   |   |   |--wake_all_kswapd(order, ...)

2.2 high watermark

达到high watermark时停止:

zone_balanced(, balance_gap:0,)  //简化问题, 忽略CONFIG_COMPACTION
|--free > high_wmark_pages+balance_gap 
|-- and ...

pgdat_balanced //简化问题, 只看order为0的情况
{
    if (zone_balanced(..,balance_gap:0, ))
        return true
}


kswapd -> balance_pgdat
{
    ...
            shrink_zone
            shrink_slab

    if (pgdat_balanced(...))
        break;      /* kswapd: all done */
}

2.3 minfreekbytes

linux-3.10.86/Documentation/sysctl/vm.txt

minfreekbytes:

This is used to force the Linux VM to keep a minimum number of kilobytes free. The VM uses this number to compute a watermark[WMARK_MIN] value for each lowmem zone in the system. Each lowmem zone gets a number of reserved free pages based proportionally on its size.

Some minimal amount of memory is needed to satisfy PF_MEMALLOC allocations; if you set this to lower than 1024KB, your system will become subtly broken, and prone to deadlock under high loads.

Setting this too high will OOM your machine instantly.

linux-3.10.86/mm/vmscan.c

pfmemalloc_watermark_ok
{
    pfmemalloc_reserve += min_wmark_pages(zone);
    wmark_ok = free_pages > pfmemalloc_reserve / 2;

    if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
        ...
        /*如果睡眠中(waitqueue_active()), 则唤醒kswapd*/
        wake_up_interruptible(&pgdat->kswapd_wait);
    }
}

https://lwn.net/Articles/422291/
The thresholds kswapd/direct reclaim starts(ends) depend on watermark[min,low,high] and currently all watermarks are set based on minfreekbytes. minfreekbytes is the amount of free memory that Linux VM should keep at least.

linux-3.10.86/mm/page_alloc.c

__setup_per_zone_wmarks
{

    zone->watermark[WMARK_MIN] = ...
    zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) + (tmp >> 2);
    zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1);
}

本文地址: https://awakening-fong.github.io/posts/mm/reclaim_04_parameter

转载请注明出处: https://awakening-fong.github.io