Clear file system cache before doing I/O-intensive benchmark on Linux

If you do any I/O-intensive benchmark, please run following command before it (Otherwise you may get wrong impression of the program performance):

$ sudo sh -c "sync; echo 3 > /proc/sys/vm/drop_caches"

sync means writing data from cache to file system (otherwise the dirty cache can’t be freed); “echo 3 > /proc/sys/vm/drop_caches” will drop clean caches, as well as reclaimable slab objects. Check following command:

$ sudo sh -c "sync; echo 3 > /proc/sys/vm/drop_caches"
$ time ./benchmark

real    0m12.434s
user    0m5.633s
sys 0m0.761s
$ time ./benchmark

real    0m6.291s
user    0m5.645s
sys 0m0.631s

the first run time of benchmark program is ~12 seconds. Now that the files are cached, the second run time of benchmark program is halved: ~6 seconds.

References:
Why drop caches in Linux?;
/proc/sys/vm;
CLEAR_CACHES.