linux mem overcommit

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 linux overcommit 机制会欺骗进程，承诺分配给他的内存空间，只有当这些page被真正touch到的时候才通过缺页机制拿到真正的物理内存。

以下摘自kernel doc

The Linux kernel supports the following overcommit handling modes0	-	Heuristic overcommit handling. Obvious overcommits ofaddress space are refused. Used for a typical system. Itensures a seriously wild allocation fails while allowingovercommit to reduce swap usage.  root is allowed to allocate slightly more memory in this mode. This is the default.1	-	Always overcommit. Appropriate for some scientificapplications. Classic example is code using sparse arraysand just relying on the virtual memory consisting almostentirely of zero pages.2	-	Don't overcommit. The total address space commitfor the system is not permitted to exceed swap + aconfigurable amount (default is 50%) of physical RAM.Depending on the amount you use, in most situationsthis means a process will not be killed while accessingpages but will receive errors on memory allocation asappropriate.Useful for applications that want to guarantee theirmemory allocations will be available in the futurewithout having to initialize every page.The overcommit policy is set via the sysctl `vm.overcommit_memory'.The overcommit amount can be set via `vm.overcommit_ratio' (percentage)
or `vm.overcommit_kbytes' (absolute value).The current overcommit limit and amount committed are viewable in
/proc/meminfo as CommitLimit and Committed_AS respectively.Gotchas
-------The C language stack growth does an implicit mremap. If you want absolute
guarantees and run close to the edge you MUST mmap your stack for the 
largest size you think you will need. For typical stack usage this does
not matter much but it's a corner case if you really really careIn mode 2 the MAP_NORESERVE flag is ignored. How It Works
------------The overcommit is based on the following rulesFor a file backed mapSHARED or READ-only	-	0 cost (the file is the map not swap)PRIVATE WRITABLE	-	size of mapping per instanceFor an anonymous or /dev/zero mapSHARED			-	size of mappingPRIVATE READ-only	-	0 cost (but of little use)PRIVATE WRITABLE	-	size of mapping per instanceAdditional accountingPages made writable copies by mmapshmfs memory drawn from the same poolStatus
------o	We account mmap memory mappings
o	We account mprotect changes in commit
o	We account mremap changes in size
o	We account brk
o	We account munmap
o	We report the commit status in /proc
o	Account and check on fork
o	Review stack handling/building on exec
o	SHMfs accounting
o	Implement actual limit enforcementTo Do
-----
o	Account ptrace pages (this is hard)

CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'),this is the total amount of  memory currently available tobe allocated on the system. This limit is only adhered toif strict overcommit accounting is enabled (mode 2 in'vm.overcommit_memory').The CommitLimit is calculated with the following formula:CommitLimit = ([total RAM pages] - [total huge TLB pages]) *overcommit_ratio / 100 + [total swap pages]For example, on a system with 1G of physical RAM and 7Gof swap with a `vm.overcommit_ratio` of 30 it wouldyield a CommitLimit of 7.3G.For more details, see the memory overcommit documentationin vm/overcommit-accounting.
Committed_AS: The amount of memory presently allocated on the system.The committed memory is a sum of all of the memory whichhas been allocated by processes, even if it has not been"used" by them as of yet. A process which malloc()'s 1Gof memory, but only touches 300M of it will show up asusing 1G. This 1G is memory which has been "committed" toby the VM and can be used at any time by the allocatingapplication. With strict overcommit enabled on the system(mode 2 in 'vm.overcommit_memory'),allocations which wouldexceed the CommitLimit (detailed above) will not be permitted.This is useful if one needs to guarantee that processes willnot fail due to lack of memory once that memory has beensuccessfully allocated.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdint.h>int main(void)
{char *temp_malloc = NULL;uint64_t i=0;printf("wait malloc\n");sleep(10);printf("start malloc\n");temp_malloc=malloc(1024*1024*30);if(temp_malloc ==NULL){   printf("malloc fail\n");return 0;}   for(i=0; i< 1024*1024*30; i++);{   temp_malloc[i] = i&0xff;}   printf("wr done\n");sleep(100);printf("free malloc\n");free(temp_malloc);return 0;
}

 如上程序以为能分配到物理内存，可是实际上free命令结果看不到used memory增加

meminfo结果对比，申请的30M都被over committed 了，所以如上程序也没有真的touch到内存？ 

 

/ # busybox pmap  2655
2655: {no such process} /mnt/mmc/sdcard/largemem
00010000       4K r-xp  /mnt/mmc/sdcard/largemem
00020000       4K r--p  /mnt/mmc/sdcard/largemem
00021000       4K rw-p  /mnt/mmc/sdcard/largemem
00022000     132K rw-p  [heap]
b50d0000   30724K rw-p    [ anon ]
b6ed1000     892K r-xp  /lib/libc-2.30.so
b6fb0000      60K ---p  /lib/libc-2.30.so
b6fbf000       8K r--p  /lib/libc-2.30.so
b6fc1000       4K rw-p  /lib/libc-2.30.so
b6fc2000      12K rw-p    [ anon ]
b6fc5000     100K r-xp  /lib/ld-2.30.so
b6feb000       8K rw-p    [ anon ]
b6fed000       4K r--p  /lib/ld-2.30.so
b6fee000       4K rw-p  /lib/ld-2.30.so
bed26000     132K rw-p  [stack]
bedd7000       4K r-xp  [sigpage]
bedd8000       4K r--p  [vvar]
bedd9000       4K r-xp  [vdso]
ffff0000       4K r-xp  [vectors]
mapped: 32108K

pmap的结果也显示出了30M的内存使用

google 查到有人用mlock 来实现防止被overcommit掉

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdint.h>
#include <sys/mman.h>int main(void)
{char *temp_malloc = NULL;uint64_t i=0;printf("wait malloc\n");sleep(10);printf("start malloc\n");temp_malloc=malloc(1024*1024*30);if(temp_malloc ==NULL){   printf("malloc fail\n");return 0;}   if(mlock(temp_malloc, 1024*1024*30)){   printf("mlock fail\n");free(temp_malloc);return 0;}   printf("wr done\n");sleep(100);printf("free malloc\n");free(temp_malloc);return 0;
}

测试结果used 果然增加了30M

/ # freetotal         used         free       shared      buffers
Mem:        352328        42864       309464            0           40
-/+ buffers:              42824       309504
Swap:            0            0            0
/ # freestart malloc
wr donetotal         used         free       shared      buffers
Mem:        352328        73576       278752            0           40
-/+ buffers:              73536       278792
Swap:            0            0            0

至于为什么真正使用了malloc的memory也会被overcommit掉的原因，需要另开一篇认真调研下

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