Effects of disk cache on application memory allocation
Since I've already promised that disk cache doesn't prevent applications from getting the memory they want, let's start with that. Here is a C app (munch.c) that gobbles up as much memory as it can, or to a specified limit:#include <stdlib.h>Running out of memory isn't fun, but the OOM killer should end just this process and hopefully the rest will be unperturbed. We'll definitely want to disable swap for this, or the app will gobble up that as well.
#include <stdio.h>
#include <string.h>
int main(int argc, char** argv) {
int max = -1;
int mb = 0;
char* buffer;
if(argc > 1)
max = atoi(argv[1]);
while((buffer=malloc(1024*1024)) != NULL && mb != max) {
memset(buffer, 0, 1024*1024);
mb++;
printf("Allocated %d MB\n", mb);
}
return 0;
}
$ sudo swapoff -aEven though it said 14MB "free", that didn't stop the application from grabbing 879MB. Afterwards, the cache is pretty empty, but it will gradually fill up again as files are read and written. Give it a try.
$ free -m
total used free shared buffers cached
Mem: 1504 1490 14 0 24 809
-/+ buffers/cache: 656 848
Swap: 0 0 0
$ gcc munch.c -o munch
$ ./munch
Allocated 1 MB
Allocated 2 MB
(...)
Allocated 877 MB
Allocated 878 MB
Allocated 879 MB
Killed
$ free -m
total used free shared buffers cached
Mem: 1504 650 854 0 1 67
-/+ buffers/cache: 581 923
Swap: 0 0 0
$
Effects of disk cache on swapping
I also said that disk cache won't cause applications to use swap. Let's try that as well, with the same 'munch' app as in the last experiment. This time we'll run it with swap on, and limit it to a few hundred megabytes:$ free -mmunch ate 400MB of ram, which was taken from the disk cache without resorting to swap. Likewise, we can fill the disk cache again and it will not start eating swap either. If you run
total used free shared buffers cached
Mem: 1504 1490 14 0 10 874
-/+ buffers/cache: 605 899
Swap: 2047 6 2041
$ ./munch 400
Allocated 1 MB
Allocated 2 MB
(...)
Allocated 399 MB
Allocated 400 MB
$ free -m
total used free shared buffers cached
Mem: 1504 1090 414 0 5 485
-/+ buffers/cache: 598 906
Swap: 2047 6 2041
watch free -m in one terminal, and find . -type f -exec cat {} + > /dev/null in another, you can see that "cached" will rise while "free" falls. After a while, it tapers off but swap is never touched1 Clearing the disk cache
For experimentation, it's very convenient to be able to drop the disk cache. For this, we can use the special file/proc/sys/vm/drop_caches. By writing 3 to it, we can clear most of the disk cache: $ free -mNotice how "buffers" and "cached" went down, free mem went up, and free+buffers/cache stayed the same.
total used free shared buffers cached
Mem: 1504 1471 33 0 36 801
-/+ buffers/cache: 633 871
Swap: 2047 6 2041
$ echo 3 | sudo tee /proc/sys/vm/drop_caches
3
$ free -m
total used free shared buffers cached
Mem: 1504 763 741 0 0 134
-/+ buffers/cache: 629 875
Swap: 2047 6 2041
Effects of disk cache on load times
Let's make two test programs, one in Python and one in Java. Python and Java both come with pretty big runtimes, which have to be loaded in order to run the application. This is a perfect scenario for disk cache to work its magic.$ cat hello.pyOur hello world apps work. Now let's drop the disk cache, and see how long it takes to run them.
print "Hello World! Love, Python"
$ cat Hello.java
class Hello {
public static void main(String[] args) throws Exception {
System.out.println("Hello World! Regards, Java");
}
}
$ javac Hello.java
$ python hello.py
Hello World! Love, Python
$ java Hello
Hello World! Regards, Java
$
$ echo 3 | sudo tee /proc/sys/vm/drop_cachesWow. 1 second for Python, and 2 seconds for Java? That's a lot just to say hello. However, now all the file required to run them will be in the disk cache so they can be fetched straight from memory. Let's try again:
3
$ time python hello.py
Hello World! Love, Python
real 0m1.026s
user 0m0.020s
sys 0m0.020s
$ time java Hello
Hello World! Regards, Java
real 0m2.174s
user 0m0.100s
sys 0m0.056s
$
$ time python hello.pyYay! Python now runs in just 22 milliseconds, while java uses 139ms. That's a 95% improvement! This works the same for every application!
Hello World! Love, Python
real 0m0.022s
user 0m0.016s
sys 0m0.008s
$ time java Hello
Hello World! Regards, Java
real 0m0.139s
user 0m0.060s
sys 0m0.028s
$
Effects of disk cache on file reading
Let's make a big file and see how disk cache affects how fast we can read it. I'm making a 200mb file, but if you have less free ram, you can adjust it.$ echo 3 | sudo tee /proc/sys/vm/drop_cachesSince the file was just written, it will go in the disk cache. The 200MB file caused a 200MB bump in "cached". Let's read it, clear the cache, and read it again to see how fast it is:
3
$ free -m
total used free shared buffers cached
Mem: 1504 546 958 0 0 85
-/+ buffers/cache: 461 1043
Swap: 2047 6 2041
$ dd if=/dev/zero of=bigfile bs=1M count=200
200+0 records in
200+0 records out
209715200 bytes (210 MB) copied, 6.66191 s, 31.5 MB/s
$ ls -lh bigfile
-rw-r--r-- 1 vidar vidar 200M 2009-04-25 12:30 bigfile
$ free -m
total used free shared buffers cached
Mem: 1504 753 750 0 0 285
-/+ buffers/cache: 468 1036
Swap: 2047 6 2041
$
$ time cat bigfile > /dev/nullThat's more than fifty times faster!
real 0m0.139s
user 0m0.008s
sys 0m0.128s
$ echo 3 | sudo tee /proc/sys/vm/drop_caches
3
$ time cat bigfile > /dev/null
real 0m8.688s
user 0m0.020s
sys 0m0.336s
$
Conclusions
The Linux disk cache is very unobtrusive. It uses spare memory to greatly increase disk access speeds, and without taking any memory away from applications. A fully used store of ram on Linux is efficient hardware use, not a warning sign.LinuxAteMyRam.com was presented by VidarHolen.net
1. This is somewhat oversimplified. While newly allocated memory will always be taken from the disk cache instead of swap, Linux can be configured to preemptively swap out other unused applications in the background to free up memory for cache. The is tunable through the 'swappiness' setting, accessible through /proc/sys/vm/swappiness.
A server might want to swap out unused apps to speed up disk access of running ones (making the system faster), while a desktop system might want to keep apps in memory to prevent lag when the user finally uses them (making the system more responsive). This is the subject of much debate.
I’m a Ubuntu guy at heart, but the team over at bicycle-based IT company 
