[gpfsug-discuss] bizarre performance behavior

[Aaron Knister]

This is a good one. I've got an NSD server with 4x 16GB fibre 
connections coming in and 1x FDR10 and 1x QDR connection going out to 
the clients. I was having a really hard time getting anything resembling 
sensible performance out of it (4-5Gb/s writes but maybe 1.2Gb/s for 
reads). The back-end is a DDN SFA12K and I *know* it can do better than 
that.

I don't remember quite how I figured this out but simply by running 
"openssl speed -multi 16" on the nsd server to drive up the load I saw 
an almost 4x performance jump which is pretty much goes against every 
sysadmin fiber in me (i.e. "drive up the cpu load with unrelated crap to 
quadruple your i/o performance").

This feels like some type of C-states frequency scaling shenanigans that 
I haven't quite ironed down yet. I booted the box with the following 
kernel parameters "intel_idle.max_cstate=0 processor.max_cstate=0" which 
didn't seem to make much of a difference. I also tried setting the 
frequency governer to userspace and setting the minimum frequency to 
2.6ghz (it's a 2.6ghz cpu). None of that really matters-- I still have 
to run something to drive up the CPU load and then performance improves.

I'm wondering if this could be an issue with the C1E state? I'm curious 
if anyone has seen anything like this. The node is a dx360 M4 
(Sandybridge) with 16 2.6GHz cores and 32GB of RAM.

-Aaron

-- 
Aaron Knister
NASA Center for Climate Simulation (Code 606.2)
Goddard Space Flight Center
(301) 286-2776