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lowmem_reserve_ratio

For some specialised workloads on highmem machines it is dangerous for
the kernel to allow process memory to be allocated from the "lowmem"
zone.  This is because that memory could then be pinned via the mlock()
system call, or by unavailability of swapspace.

And on large highmem machines this lack of reclaimable lowmem memory
can be fatal.

So the Linux page allocator has a mechanism which prevents allocations
which _could_ use highmem from using too much lowmem.  This means that
a certain amount of lowmem is defended from the possibility of being
captured into pinned user memory.

(The same argument applies to the old 16 megabyte ISA DMA region.  This
mechanism will also defend that region from allocations which could use
highmem or lowmem).

The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
in defending these lower zones.

If you have a machine which uses highmem or ISA DMA and your
applications are using mlock(), or if you are running with no swap then
you probably should change the lowmem_reserve_ratio setting.

The lowmem_reserve_ratio is an array. You can see them by reading this file.
-
% cat /proc/sys/vm/lowmem_reserve_ratio
256     256     32
-
Note: # of this elements is one fewer than number of zones. Because the highest
      zone's value is not necessary for following calculation.

But, these values are not used directly. The kernel calculates # of protection
pages for each zones from them. These are shown as array of protection pages
in /proc/zoneinfo like followings. (This is an example of x86-64 box).
Each zone has an array of protection pages like this.

-
Node 0, zone      DMA
  pages free     1355
        min      3
        low      3
        high     4
	:
	:
    numa_other   0
        protection: (0, 2004, 2004, 2004)
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  pagesets
    cpu: 0 pcp: 0
        :
-
These protections are added to score to judge whether this zone should be used
for page allocation or should be reclaimed.

In this example, if normal pages (index=2) are required to this DMA zone and
watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
not be used because pages_free(1355) is smaller than watermark + protection[2]
(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
normal page requirement. If requirement is DMA zone(index=0), protection[0]
(=0) is used.

zone[i]'s protection[j] is calculated by following expression.

(i < j):
  zone[i]->protection[j]
  = (total sums of present_pages from zone[i+1] to zone[j] on the node)
    / lowmem_reserve_ratio[i];
(i = j):
   (should not be protected. = 0;
(i > j):
   (not necessary, but looks 0)

The default values of lowmem_reserve_ratio[i] are
    256 (if zone[i] means DMA or DMA32 zone)
    32  (others).
As above expression, they are reciprocal number of ratio.
256 means 1/256. # of protection pages becomes about "0.39%" of total present
pages of higher zones on the node.

If you would like to protect more pages, smaller values are effective.
The minimum value is 1 (1/1 -> 100%).

Quellcode

http://www.kernel.org/doc/Documentation/sysctl/vm.txt

Quellcode

Aug 21 12:32:54 server kernel: [2652848.264806] Node 0 DMA free:15916kB min:28kB low:32kB high:40kB active_anon:0kB inactive_anon:0kB active_file:8kB inactive_file:0kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:15668kB mlocked:0kB dirty:0kB writeback:0kB mapped:0kB shmem:0kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:0kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
Aug 21 12:32:54 server kernel: [2652848.264815] lowmem_reserve[]: 0 3469 3973 3973
Aug 21 12:32:54 server kernel: [2652848.264818] Node 0 DMA32 free:91288kB min:7028kB low:8784kB high:10540kB active_anon:99356kB inactive_anon:97900kB active_file:1314264kB inactive_file:1421100kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:3553024kB mlocked:0kB dirty:202412kB writeback:62748kB mapped:376420kB shmem:268kB slab_reclaimable:126304kB slab_unreclaimable:23652kB kernel_stack:496kB pagetables:7136kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
Aug 21 12:32:54 server kernel: [2652848.264828] lowmem_reserve[]: 0 0 504 504
Aug 21 12:32:54 server kernel: [2652848.264831] Node 0 Normal free:1196kB min:1020kB low:1272kB high:1528kB active_anon:19568kB inactive_anon:35992kB active_file:57788kB inactive_file:58612kB unevictable:0kB isolated(anon):0kB isolated(file):128kB present:516096kB mlocked:0kB dirty:16700kB writeback:12572kB mapped:219376kB shmem:3440kB slab_reclaimable:18624kB slab_unreclaimable:15720kB kernel_stack:1056kB pagetables:4168kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:286 all_unreclaimable? no
Aug 21 12:32:54 server kernel: [2652848.264841] lowmem_reserve[]: 0 0 0 0

Quellcode

free -m
         	total   	used   	free 	shared	buffers 	cached
Mem:      	3042   	1396   	1645      	0     	47    	662
-/+ buffers/cache:    	687   	2354
Swap:      	561      	0    	561