I have observed that the %si usage in the system goes to high values (10%) (in top output) with just with two ftp sessions from different external machines happening to the server.

The server configuration includes a 16 core CPU with 8 GB RAM. The ftp sessions are happening over IPSec tunnels.

The same is not observed when the FTP happens from the same external machine.

I am looking ways to to analyse which softirqs are taking more time so that we can optimize the usage. Please suggest.

Are there any kernel flags which I can switch on to get some statistical data about the softirq and get more insight into the operation?