Hardware encryption actually increases CPU utilization?

[rhester72]

I didn't want to necro-thread (http://archlinuxarm.org/forum/viewtopic.php?f=30&t=2452&start=70), but I am trying to understand this...

Using OpenVPN server on my Pogoplug E02, I see somewhere in the neighborhood of 15% CPU utilization if I ignore Kirkwood encryption hardware acceleration and try to max out my OpenVPN bandwidth (via cable modem).

If I then enable Kirkwood encryption (via openssl-cryptodev, plus "engine cryptodev" and "cipher AES-128-CBC" options in my OpenVPN server config), I can see the interrupts incrementing rapidly in /proc/crypto (so I'm confident I am in fact using Kirkwood's encryption offload), but throughput remains about the same as without hardware accel (not _too_ surprising) and CPU utilization increases to 100% (which is shocking and exactly the opposite of what I expected). As soon as the I/O load through OpenVPN approaches zero, CPU utilization goes flat as well.

Anyone seen anything like this, where HW crypto actually uses *more* processor resources than without?

Rodney