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Full Discussion: Linux Virtual Memory Question
Top Forums UNIX for Advanced & Expert Users Linux Virtual Memory Question Post 302560798 by achenle on Saturday 1st of October 2011 08:58:33 AM
Old 10-01-2011
Quote:
Originally Posted by Corona688
I beg to differ:

Code:
$ cat megaalloc.c
#include <stdio.h>
#include <unistd.h>

int main(void)
{
    sbrk((1024L*1024L*1024L));
    while(1);
}
$ gcc megaalloc.c
$ ./a.out

Code:
top - 17:46:55 up  6:12,  4 users,  load average: 0.24, 0.17, 0.11
Tasks: 127 total,   2 running, 124 sleeping,   0 stopped,   1 zombie
Cpu(s): 45.5%us,  3.5%sy,  0.0%ni, 48.2%id,  0.3%wa,  2.5%hi,  0.0%si,  0.0%st
Mem:   3797432k total,  3236628k used,   560804k free,   161272k buffers
Swap:  2104476k total,        0k used,  2104476k free,  1373932k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
10599 monttyle  20   0 1027m  296  228 R   73  0.0   0:02.19 a.out              
 3825 root      20   0  748m  82m  10m S    8  2.2  25:52.57 X                  
10017 monttyle  20   0  149m 8032 5936 S    8  0.2   3:49.15 mplayer            
 9712 monttyle  20   0  220m  15m  10m S    5  0.4   4:38.51 gnome-terminal     
 5988 monttyle  20   0 1591m 1.3g  26m S    3 37.0  52:49.71 firefox-bin        
 9710 monttyle  20   0 71692  13m 4900 S    2  0.4   3:03.59 wish               
 5256 monttyle  20   0  216m  14m  10m S    1  0.4   2:54.01 metacity           
    8 root      20   0     0    0    0 S    0  0.0   0:10.05 events/1           
  453 root      20   0     0    0    0 S    0  0.0   1:20.78 kondemand/1        
 5278 monttyle  20   0  346m  23m  15m S    0  0.6   4:30.59 gnome-panel        
    1 root      20   0  3892  620  516 S    0  0.0   0:01.42 init               
    2 root      20   0     0    0    0 S    0  0.0   0:00.00 kthreadd           
    3 root      RT   0     0    0    0 S    0  0.0   0:00.06 migration/0        
    4 root      20   0     0    0    0 S    0  0.0   0:00.42 ksoftirqd/0        
    5 root      RT   0     0    0    0 S    0  0.0   0:00.01 migration/1        
    6 root      20   0     0    0    0 S    0  0.0   0:02.82 ksoftirqd/1        
    7 root      20   0     0    0    0 S    0  0.0   0:07.20 events/0

My process certainly does not have 1027M resident. It doesn't have it on disk, either -- having not used any of that gig of ram, none of it is actually paged in, resident, or even backed in any form. Other systems might demand sufficient swap to back it but not Linux, which by default has virtual swap.
Linux doesn't have "virtual swap", but by default Linux does overcommit swap space - IMO a REALLY, REALLY BAD IDEA.

Because eventually it's going to get used. Which means the OOM killer is going to wake up and start randomly killing processes, aiming for heavy memory users.

You know - the processes like web servers and databases that are the actual reason you're using the computer in the first place.

"Works most of the time, then goes BOOM!" is an awfully low reliability standard.

Quote:
On a 64-bit system you can also inflate that value by plunking in your entire hard disk as a memory segment via mmap. 300G virtual. I suppose that would count as on disk though.
That wouldn't usually need swap space because the backing store for the virtual memory would be your hard disk you mmap()'d, but you could probably map it in such a way that if you were to write to it, the changed pages would require swap backing. Maybe by mmap()'ing a read-only file descriptor read/write?
 

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