How to reduce free memory to certain number for specific time duration

Question

I want to stress memory for certain time duration. I am checking memory usage with vmstat -s and using tail /dev/zero command but tail /dev/zero times out in about 60s and it fills out memory randomly. I want to have available free memory 5% for 180 seconds.

Answer 1

One could use the kernel trace buffer as a way to reduce available memory left for the system and/or user. Example, using my system with 16 gigabytes and 8 CPUs:

First flush memory, giving our starting point (note that I am running as root):

root@s15:/home/doug/temp-k-git/linux# sync
root@s15:/home/doug/temp-k-git/linux# echo 3 > /proc/sys/vm/drop_caches
root@s15:/home/doug/temp-k-git/linux# free -m
              total        used        free      shared  buff/cache   available
Mem:          15719          88       15472           1         159       15343
Swap:         16085          35       16050

Now, calculate and allocate the memory to the kernel trace buffer: 5% of 15719 megabytyes is 786 megabytes. Starting from 15472 megabytyes, Then 14686 megabyytes need to be used. The kernel trace buffer is per CPU, in my case 8 CPUs, so 1835 megabytes per CPU. Allocate:

root@s15:/home/doug/temp-k-git/linux# echo 1835000 > /sys/kernel/debug/tracing/buffer_size_kb

And check it:

root@s15:/home/doug/temp-k-git/linux# free -m
              total        used        free      shared  buff/cache   available
Mem:          15719       14476         858           1         385         730
Swap:         16085          35       16050

You can observe that it resulted in 5.46% of memory still available (tweek the allocation, if needed). Once you have done whatever testing, then you can de-allocate the kernel trace buffer with (0 does not work):

root@s15:/home/doug/temp-k-git/linux# echo 1 > /sys/kernel/debug/tracing/buffer_size_kb

And check it:

root@s15:/home/doug/temp-k-git/linux# free -m
              total        used        free      shared  buff/cache   available
Mem:          15719          88       15470           1         160       15342
Swap:         16085          35       16050

If you want the time to be 180 seconds, then bundle up the above into a script with a 180 second sleep time. However, you will still have to check the available number as your starting point.