man ksoftirqd indicates that: If ksoftirqd is taking more than a tiny percentage of CPU time, this  indicates the machine is under heavy soft interrupt load.I'm working with a Debian Wheezy system under generally high system utilization in which ksoftirqd processes utilizes excessive cpu and disk resources for a short period of time. During that time, the system operates at a snails pace.How can one begin to understand what the root cause is for this ksoftirqd resource utilization spikes? 
How to debug causes of excessive ksoftirqd resource usage?
interrupt;load
Check /proc/interrupts to find if one of or more interrupts occur excessively. Hint: Several thousand interrupts per second are no cause for alarm.Excessive interrupts (aka interrupt storms) can have multiple reasons, one of them even being hardware issues (noisy interrupt line).To further answer your question we need to know what OS on what hardware you use.