[Beowulf] First experiences with Broadwell and the Dell M630

John Hearns hearnsj at googlemail.com
Thu Jun 9 22:54:49 PDT 2016

I have 2650-v4 processors in nodes with 128 Gbytes of RAM (not Dell though)
should anyone be interested in comparisions.
I should say I have 2650-v3 available for comparison also.
One thing I found recently, which is no surprise really, when running
Openfoam on a 2650-v4,
I was looking at the influence of CPU core binding with OpenMPI.
I found around a 10% gain  - but on one node the performance was
consistently 5% WORSE.
It turned out this node had a fan failure (one of three fans).
As I say its no surprise, but it makes sense that processors of this
generation are sensitive to cooling as they have turboboost.
That maybe chimes with what you see Bill in the variability?

Might be a good experiment to deliberately disconnect fans and see the
performance effect with HPL.

On 10 June 2016 at 06:46, John Hearns <hearnsj at googlemail.com> wrote:

> Bill
> I have other types of servers with 2650-v4 and 128 Gbytes RAM.
> If it is any help I can run tests with various BIOS settings also.
> Can take discussion offline.
> On 10 June 2016 at 02:26, Bill Wichser <bill at princeton.edu> wrote:
>> 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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