How to fill 90% of free memory?

My first "tech job" was doing support in the late 90s for DOS games at EA. At that point most folks had computers with 8 or 16MB of RAM. One of the old Jane's Flight simulator games had a check for minimum RAM requirements which for some reason would overflow at 128MB and say you didn't meet minimum memory requirements. So these folks would spend around $1000 on memory to build premium machines with 128MB and then get these old games which would say they couldn't play the game due to insufficient memory. The fix was to create a RAM drive which would caption of part of the RAM available to storage and leave a limited amount of available RAM to run the game which would clear the memory validation.

I believe at one point someone was able to actually install a game into the RAM Drive and play it on the left over available RAM but it required a game that could be installed and played without a reboot.

http://en.wikipedia.org/wiki/RAM_drive

Infinite loop: HN links to this question, StackExchange links to HN for an answer.

Just fill /dev/shm via dd or similar.

    dd if=/dev/zero of=/dev/shm/fill bs=1k count=1024k

(option #2 is to limit the amount of ram available to the kernel via grub.conf, see my comment below)

I recently recorded a screencast [1], about linux cgroups, and how you can restrict/shape various program resources. For one of the examples, I wrote the following C program to take 50MB of memory, at around the 15:45 mark in the screencast you can see it in action, but this could easy be modified, and add a sleep, to hold the memory for a while. You would most likely need to disable swap if you wanted to use 90% of the free memory, run "free -m; swapoff -a; free -m" as root, then use "swapon -a" to enable it again.

  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  
  int main(void) {

      int i;
      char *p;

      /* intro message */
      printf("Starting ...\n");

      /* loop 50 times, try and consume 50 MB of memory */
      for (i = 0; i < 50; ++i) {

          /* failure to allocate memory? */
          if ((p = malloc(1<<20)) == NULL) {
              printf("Malloc failed at %d MB\n", i);
              return 0;
          }

          /* take memory and tell user where we are at */
          memset(p, 0, (1<<20));
          printf("Allocated %d to %d MB\n", i, i+1);

      }

      /* exit message and return */
      printf("Done!\n");
      return 0;

  }

[1] http://sysadmincasts.com/episodes/14-introduction-to-linux-c...

Or you could use 'stress':

http://linux.die.net/man/1/stress

Just run several instances of Firefox for a couple of days non stop

I'm genuinely curious, why is this being posted to HN? This seems like something any good systems programmer (i.e. C on UNIX) would know and I'm sure there are plenty of people like that on StackExchange.

I answered a very similar question on stackoverflow here:

http://stackoverflow.com/q/1229241/25981

In this case the user wanted the program to run out of memory and the best solution I could come up with was to use ulimit, to limit the amount of memory available to the process.

Option 1: tmpfs or another memory backed fs

Option 2: Quick C program, but two gotchas: make sure you touch every page after allocating (and keep touching it, otherwise they will be swapped out)

(Also turning off or limiting swap space may be helpful)

Virtualbox running a Windows VM seems pretty good at clinging onto memory and not swapping out. You also get a nice little graphical slider to determine how much memory is allocated to a given VM.

Zeno's little known memory paradox?

Nowadays I would just run Linux in a VirtualBox configured with the amount of RAM that I wanted to simulate. I've done the same thing with CPU cores to compare performance with 1, 2, 4 and 8 cores. Of course I run VirtualBox on a 16-core server...

I'd wonder whether methods that make use of zeroing or /dev/zero'd be staved off for longer on Mavericks - perhaps it'd compress the recurring patterns that'd result?

Would a malloc (alone) work? Doesn't it typically act as if the memory were allocated but not actually use physical RAM until the data is filled?

A java hello world application should do the trick.