[Beowulf] First experiences with Broadwell and the Dell M630

[Bill Wichser]

You'd think that by now I'd know better.  Trying to live on the cutting 
edge.  But the promise of 5% over Haswell was quite alluring.

We purchased Broadwell 120W 2680v4 chips with 128G of RAM enclosed in 
the Dell M630 blades.  When we finally received power the first thing we 
did was load a RHEL7 OS, checked BIOS to be sure we had all the 
performance variables set and ran HPL compiled with Intel v16 compilers 
against their MKL.  Performance went from a high of 791 GFLOPS down to 
679.  A whopping 14% difference.

Test      element        low       high          % mean median    std dev
WR00L2L4               315     679.44     791.34 14.14 741.67     
739.64      19.89

We should be able to do better than this and reduce to something like 5% 
right?

We checked power settings for the chassis and played with those.  we 
turned power management to BIOS and then to the OS using ACPI.  No 
difference.  We swapped the fastest and slowest nodes thinking that this 
might be a location issue.  No difference.  And then we found a BIOS 
update from 2.0.1 to 2.1.6 which was fresh so loaded that one up.

Performance went down.  Considerably.

Test      chassis        low       high          %       mean median    
std dev
WR00L2L4      296     583.47     636.27       8.30     623.74 
625.15       9.82

Wow!  741 to 623 GFLOPS!

We then looked at power and heat using the turbostat command to log 
values.  What we found was that at the slowest nodes the SMI interrupts 
and c1 states were higher and the power was capped at 120W.  On the 
fastest nodes, things were different with power hovering around 117W.  
Again switching node slots changed nothing.

With Dell's help we finally manged to turn off turbo mode and set the 
--ProcConfigTdp=Level1 to only run at the base AVX speed of 1.9GHz.  
This indeed provided much closer HPL results.

Test      chassis        low       high          %       mean median    
std dev
WR00L2L4      310     515.95     519.55       0.69     519.07 
519.14       0.34

with plenty of the nodes hovering around the 110W usage.

But now we had a new motherboard and with the same setup ran another 
test, this time without updating BIOS so it still was sitting on 2.0.1.  
Lo and behold, there's that (weak) performance again.  638 GFLOPS and 
better power usage.

   638.12, Temps:   78,   70, Watts:   107.02,   104.65

We still don't understand what we are up against here.  Obviously 
re-enabling the performance variables in BIOS will begin to get those 
FLOPS up again.  As will degrading the BIOS and microcode. And maybe 
this 14% difference between best and worst nodes is all we can expect.  
But I'd sure like to have a lot more of those better performers!

Did I mention GPFS?  We have it running on a v3 node with the same 
kernel.  On the Broadwell chips though, it just hangs the kernel. Sigh.  
The cutting edge.  When can I order Skylake?

Bill
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