减少竞争 per_cpu_pageset
在伙伴系统的研究中，我们看到page是挂在对应的zone下面的，这个数量是固定的。如果我们每次直接从伙伴系统中获取页，那会导致一个问题。
随着cpu个数的增加，对伙伴系统的竞争也会越来越大
linux内核中为了解决这个问题，引入了per_cpu_pageset。
其实理念很简单，就是先从zone这个大仓库里拿一些页出来，放到每个cpu自己的小仓库。用好了，也先放回到这里，等满了再一起还给zone。
那接下来先看看这个结构在zone中是什么样子的。
struct zone
+------------------------------------------------------------------------------------------------+
|pageset |
| (struct per_cpu_pageset *) |
| cpu0 cpu1 cpuN |
| +--------------------------+ +--------------------------+ ... +--------------------------+
| |pcp | |pcp | |pcp |
| | (struct per_cpu_pages) | | (struct per_cpu_pages) | | (struct per_cpu_pages) |
| | +-----------------------+ | +-----------------------+ | +-----------------------+
| | |count | | |count | | |count |
| | |high | | |high | | |high |
| | |batch | | |batch | | |batch |
| | | | | | | | | |
| | |lists[MIGRATE_PCPTYPES]| | |lists[MIGRATE_PCPTYPES]| | |lists[MIGRATE_PCPTYPES]|
+---+--+-----------------------+--+--+-----------------------+--------+--+-----------------------+可以看到，对于任意一个zone都记录了每个cpu上那个小仓库的信息。具体到每个成员的含义为：
count: 当前小仓库中有多少页
high: 如果小仓库里有超过high个数量的页，则还页面到zone
batch: 如果小仓库没有页面了，则一次从zone中找batch个页面来
这些值还能通过/proc/percpu_pagelist_fraction来动态调节。
那系统中是否能够观察到这些数值呢？也是有办法的。通过/proc/zoneinfo就可以。不过这个文件有点大，大家要仔细去看pageset的字段。
我在这里给大家整理出来一个4个node，8个cpu系统上的zoneinfo中pageset的信息。这样或许有个直观的理解。
Node 0, zone DMA Node 0, zone DMA32 Node 0, zone Normal Node 0, zone Movable
pagesets pagesets
cpu: 0 cpu: 0
count: 0 count: 299
high: 0 high: 378
batch: 1 batch: 63
vm stats threshold: 8 vm stats threshold: 40
cpu: 1 cpu: 1
count: 0 count: 86
high: 0 high: 378
batch: 1 batch: 63
vm stats threshold: 8 vm stats threshold: 40
cpu: 2 cpu: 2
count: 0 count: 298
high: 0 high: 378
batch: 1 batch: 63
vm stats threshold: 8 vm stats threshold: 40
cpu: 3 cpu: 3
count: 0 count: 0
high: 0 high: 378
batch: 1 batch: 63
vm stats threshold: 8 vm stats threshold: 40
cpu: 4 cpu: 4
count: 0 count: 0
high: 0 high: 378
batch: 1 batch: 63
vm stats threshold: 8 vm stats threshold: 40
cpu: 5 cpu: 5
count: 0 count: 33
high: 0 high: 378
batch: 1 batch: 63
vm stats threshold: 8 vm stats threshold: 40
cpu: 6 cpu: 6
count: 0 count: 7
high: 0 high: 378
batch: 1 batch: 63
vm stats threshold: 8 vm stats threshold: 40
cpu: 7 cpu: 7
count: 0 count: 0
high: 0 high: 378
batch: 1 batch: 63
vm stats threshold: 8 vm stats threshold: 40
Node 1, zone DMA Node 1, zone DMA32 Node 1, zone Normal Node 1, zone Movable
pagesets pagesets
cpu: 0 cpu: 0
count: 0 count: 16
high: 378 high: 378
batch: 63 batch: 63
vm stats threshold: 32 vm stats threshold: 32
cpu: 1 cpu: 1
count: 0 count: 6
high: 378 high: 378
batch: 63 batch: 63
vm stats threshold: 32 vm stats threshold: 32
cpu: 2 cpu: 2
count: 59 count: 259
high: 378 high: 378
batch: 63 batch: 63
vm stats threshold: 32 vm stats threshold: 32
cpu: 3 cpu: 3
count: 61 count: 329
high: 378 high: 378
batch: 63 batch: 63
vm stats threshold: 32 vm stats threshold: 32
cpu: 4 cpu: 4
count: 0 count: 5
high: 378 high: 378
batch: 63 batch: 63
vm stats threshold: 32 vm stats threshold: 32
cpu: 5 cpu: 5
count: 0 count: 125
high: 378 high: 378
batch: 63 batch: 63
vm stats threshold: 32 vm stats threshold: 32
cpu: 6 cpu: 6
count: 0 count: 110
high: 378 high: 378
batch: 63 batch: 63
vm stats threshold: 32 vm stats threshold: 32
cpu: 7 cpu: 7
count: 0 count: 272
high: 378 high: 378
batch: 63 batch: 63
vm stats threshold: 32 vm stats threshold: 32
Node 2, zone DMA Node 2, zone DMA32 Node 2, zone Normal Node 2, zone Movable
pagesets
cpu: 0
count: 165
high: 378
batch: 63
vm stats threshold: 40
cpu: 1
count: 175
high: 378
batch: 63
vm stats threshold: 40
cpu: 2
count: 291
high: 378
batch: 63
vm stats threshold: 40
cpu: 3
count: 137
high: 378
batch: 63
vm stats threshold: 40
cpu: 4
count: 122
high: 378
batch: 63
vm stats threshold: 40
cpu: 5
count: 362
high: 378
batch: 63
vm stats threshold: 40
cpu: 6
count: 322
high: 378
batch: 63
vm stats threshold: 40
cpu: 7
count: 198
high: 378
batch: 63
vm stats threshold: 40
Node 3, zone DMA Node 3, zone DMA32 Node 3, zone Normal Node 3, zone Movable
pagesets
cpu: 0
count: 206
high: 378
batch: 63
vm stats threshold: 40
cpu: 1
count: 354
high: 378
batch: 63
vm stats threshold: 40
cpu: 2
count: 271
high: 378
batch: 63
vm stats threshold: 40
cpu: 3
count: 0
high: 378
batch: 63
vm stats threshold: 40
cpu: 4
count: 139
high: 378
batch: 63
vm stats threshold: 40
cpu: 5
count: 325
high: 378
batch: 63
vm stats threshold: 40
cpu: 6
count: 135
high: 378
batch: 63
vm stats threshold: 40
cpu: 7
count: 332
high: 378
batch: 63
vm stats threshold: 40Last updated
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