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| -rw-r--r-- | Documentation/filesystems/proc.txt | 80 |
1 files changed, 63 insertions, 17 deletions
diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt index 07231afca72..e2799b5fafe 100644 --- a/Documentation/filesystems/proc.txt +++ b/Documentation/filesystems/proc.txt @@ -1336,7 +1336,7 @@ legacy_va_layout If non-zero, this sysctl disables the new 32-bit mmap mmap layout - the kernel will use the legacy (2.4) layout for all processes. -lower_zone_protection +lowmem_reserve_ratio --------------------- For some specialised workloads on highmem machines it is dangerous for @@ -1356,25 +1356,71 @@ captured into pinned user memory. mechanism will also defend that region from allocations which could use highmem or lowmem). -The `lower_zone_protection' tunable determines how aggressive the kernel is -in defending these lower zones. The default value is zero - no -protection at all. +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 increase the lower_zone_protection setting. - -The units of this tunable are fairly vague. It is approximately equal -to "megabytes," so setting lower_zone_protection=100 will protect around 100 -megabytes of the lowmem zone from user allocations. It will also make -those 100 megabytes unavailable for use by applications and by -pagecache, so there is a cost. - -The effects of this tunable may be observed by monitoring -/proc/meminfo:LowFree. Write a single huge file and observe the point -at which LowFree ceases to fall. - -A reasonable value for lower_zone_protection is 100. +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 +pages_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 exprssion. + +(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%). page-cluster ------------ |