Cold-start-simulator

Linux / Cold start simulator

This is a very non-perfect simulation. Indeed - unfortunately - results vary by ~30%. However - it certainly slows down application startup under Linux - giving far more weight to cold-start issues, and a far better idea of the proportion of time spent in I/O.

Flushing Linux Caches

Recent versions of Linux (i.e. since 2.6.16) have added the ability to flush their caches. You can run:

sync ; echo 3 | sudo tee /proc/sys/vm/drop_caches

to write data to disk and then drop pagecache, dentry and inode caches. See [1] for more details.

I've not tested it with OpenOffice, but other when benchmarking other programs I get very low variance suggesting that it "does exactly what it says on the tin".

FreeBSD don't seem to provide a similar tool; with the people on IRC insisting that such a feature would never be useful. Ah well, maybe somebody will convince them.

Original tooling from Robert Love:

Using it

gcc fillmem.c -o fillmem
gcc flushdisk.c -o flushdisk
./fillmem `free -m | grep ^Mem: | awk '{print $2}'`
for i in `mount | grep ^/dev | awk '{print $1}'`; do ./flushdisk $i; done

Of course - this has the issue that while you evict pages they are potentially being required to re-render the terminal - make various things respond etc. Running the last 2 lines several times is perhaps better.

After running this it is probably wise to run & close a simple app - eg. 'gedit' that will touch all of the common libraries, X11, gtk+, fontconfig etc. and warm up the files commonly in the desktop working set.

fillmem

/*
 * fillmem.c - fill all memory to force the eviction of page cache
 *
 * Robert Love <rml@novell.com>
 */
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
 
#define MB      (1024 * 1024)
 
int main (int argc, char *argv[])
{
        unsigned long nr, i;
 
        if (argc < 2) {
                fprintf (stderr, "usage: %s <memory in MB>\n", argv[0]);
                return 1;
        }
 
        errno = 0;
        nr = strtoul (argv[1], NULL, 0);
        if (errno) {
                perror ("strtoul");
                return 1;
        }
 
        for (i = 0; i < nr; i++) {
                unsigned long *data;
 
                data = malloc (MB);
                if (!data) {
                        perror ("malloc");
                        return 1;
                }
                memset (data, i, MB);
                printf ("%p filled with %ld\n", data, i);
        }
 
        printf ("done\n");
        return 0;
}

flushdisk

/*
 * flushdisk.c - flush the buffer cache of a given blkdev
 *
 * Robert "Spunky" Love <rml@novell.com>
 */
 
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/fs.h>
 
int main (int argc, char *argv[])
{
        char *blkdev;
        int fd, ret;
 
        if (argc < 2) {
                fprintf (stderr, "usage: %s <disk to flush>\n", argv[0]);
                return 1;
        }
 
        blkdev = argv[1];
 
        fd = open (blkdev, O_RDONLY);
        if (fd < 0) {
                perror ("open");
                return 1;
        }
 
        ret = ioctl (fd, BLKFLSBUF);
        if (ret) {
                perror ("ioctl");
                return 1;
        }
 
        return 0;
}