How to avoid caching effects in read benchmarks

Question

I have a read benchmark and between consecutive runs, I have to make sure that the data does not reside in memory to avoid effects seen due to caching. So far what I used to do is: run a program that writes a large file between consecutive runs of the read benchmark. Something like

./read_benchmark
./write --size 64G --path /tmp/test.out
./read_benchmark

The write program simply writes an array of size 1G 64 times to file. Since the size of the main memory is 64G, I write a file that is approx. the same size. The problem is that writing takes a long time and I was wondering if there are better ways to do this, i.e. avoid effects seen when data is cached.

Also, what happens if I write data to /dev/null?

./write --size 64G --path /dev/null

This way, the write program exits very fast, no I/O is actually performed, but I am not sure if it overwrites 64G of main memory, which is what I ultimately want.

Your input is greatly appreciated.

Answer 1

You can drop all caches using a special file in /proc like this:

echo 3 > /proc/sys/vm/drop_caches

That should make sure cache does not affect the benchmark.

Answer 2

You can just unmount the filesystem and mount it back. Unmounting flushes and drops the cache for the filesystem.