high CPU usage = better ping results?!

Gabi Souza · 01-29-2013, 10:38 AM

Hello,
I’m working on some measurements and the ping results are intriguing me. I tested it in several scenarios and the results are the same: the ping results are better when the CPU load is higher.

The Round Trip Time decreases from about 30 usec with low CPU to ~5 or less usec with high CPU. In a quadcore machine, a single CPU being used already causes this effect

Anyone has seen this kind of behavior before? It seems to be scheduling of the Linux kernel but I’m not sure.

Thanks!

jefro · 01-30-2013, 05:04 PM

The only thing I could guess is that somehow part of the process gets offloaded to the nic. And I don't know how the kernel can change that based on cpu load. Usually it is a nic setting to offload checksum to nic. That tends to be the fastest way to set for server work.

As you say, it could be some oddity of numbers, where the process actually gets a skewed priority.

whizje · 01-30-2013, 07:22 PM

It might be that at low load the processor goes to a lower power mode at the expense of higher latencies.

jefro · 01-31-2013, 03:33 PM

A good thing to check also is what whizie suggests.