[gpfsug-discuss] bizarre performance behavior

Thu Apr 20 16:07:32 BST 2017

Hi Kennmeth, 

is prefetching off or on  at your storage backend?
Raw sequential is very different from GPFS sequential at the storage 
device !
GPFS does its own prefetching, the storage would never know what sectors 
sequential read at GPFS level maps to at storage level!

Mit freundlichen Grüßen / Kind regards

Dr. Uwe Falke

IT Specialist
High Performance Computing Services / Integrated Technology Services / 
Data Center Services
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From:   Kenneth Waegeman <kenneth.waegeman at ugent.be>
To:     gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date:   04/20/2017 04:53 PM
Subject:        Re: [gpfsug-discuss] bizarre performance behavior
Sent by:        gpfsug-discuss-bounces at spectrumscale.org

Hi,

Having an issue that looks the same as this one: 
We can do sequential writes to the filesystem at 7,8 GB/s total , which is 
the expected speed for our current storage    
backend.  While we have even better performance with sequential reads on 
raw storage LUNS, using GPFS we can only reach 1GB/s in total (each nsd 
server seems limited by 0,5GB/s) independent of the number of clients   
(1,2,4,..) or ways we tested (fio,dd). We played with blockdev params, 
MaxMBps, PrefetchThreads, hyperthreading, c1e/cstates, .. as discussed in 
this thread, but nothing seems to impact this read performance. 
Any ideas?
Thanks!

Kenneth

On 17/02/17 19:29, Jan-Frode Myklebust wrote:
I just had a similar experience from a sandisk infiniflash system 
SAS-attached to s single host. Gpfsperf reported 3,2 Gbyte/s for writes. 
and 250-300 Mbyte/s on sequential reads!! Random reads were on the order 
of 2 Gbyte/s.

After a bit head scratching snd fumbling around I found out that reducing 
maxMBpS from 10000 to 100 fixed the problem! Digging further I found that 
reducing prefetchThreads from default=72 to 32 also fixed it, while 
leaving maxMBpS at 10000. Can now also read at 3,2 GByte/s.

Could something like this be the problem on your box as well?

-jf
fre. 17. feb. 2017 kl. 18.13 skrev Aaron Knister <aaron.s.knister at nasa.gov
>:
Well, I'm somewhat scrounging for hardware. This is in our test
environment :) And yep, it's got the 2U gpu-tray in it although even
without the riser it has 2 PCIe slots onboard (excluding the on-board
dual-port mezz card) so I think it would make a fine NSD server even
without the riser.

-Aaron

On 2/17/17 11:43 AM, Simon Thompson (Research Computing - IT Services)
wrote:
> Maybe its related to interrupt handlers somehow? You drive the load up 
on one socket, you push all the interrupt handling to the other socket 
where the fabric card is attached?
>
> Dunno ... (Though I am intrigued you use idataplex nodes as NSD servers, 
I assume its some 2U gpu-tray riser one or something !)
>
> Simon
> ________________________________________
> From: gpfsug-discuss-bounces at spectrumscale.org [
gpfsug-discuss-bounces at spectrumscale.org] on behalf of Aaron Knister [
aaron.s.knister at nasa.gov]
> Sent: 17 February 2017 15:52
> To: gpfsug main discussion list
> Subject: [gpfsug-discuss] bizarre performance behavior
>
> 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
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--
Aaron Knister
NASA Center for Climate Simulation (Code 606.2)
Goddard Space Flight Center
(301) 286-2776
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