Tuning InfiniBand Interconnects Using Congestion Control

By Adam Dorsey

July 26, 2017

InfiniBand is among the most common and well-known cluster interconnect technologies. However, the complexities of an InfiniBand (IB) network can frustrate the most experienced cluster administrators. Maintaining a balanced fabric topology, dealing with underperforming hosts and links, and chasing potential improvements keeps all of us on our toes. Sometimes, though, a little research and experimentation can find unexpected performance and stability gains.

For example, consider a 1,300-node cluster using Intel TrueScale IB for job communication and a Panasas ActiveStor filesystem for storage. Panasas only communicates to clients via Ethernet and not IB, so a group of Mellanox switches act as gateways from the Panasas Ethernet to the TrueScale IB.

Every system has bottlenecks; in our case, the links to and from these IB/Ethernet gateways showed congestion due to the large amount of disk traffic. This adversely affects the whole cluster — jobs can’t get the data they need, and the increased congestion interferes with other IB traffic as well.

Fortunately, InfiniBand provides a congestion control mechanism that can help mitigate the effects of severe congestion on the fabric. We were able to implement this feature to save the expense and trouble of adding additional IB/Ethernet gateways.

What Is InfiniBand Congestion Control?

InfiniBand is intended to be a lossless fabric. IB switches won’t drop packets for flow control unless they absolutely have to, usually in cases of hardware failure or malformed packets. Instead of dropping packets and retransmitting, like Ethernet does, InfiniBand uses a system of credits to perform flow control.

Communication occurs between IB endpoints, which in turn are issued credits based on the amount of buffer space the receiving device has. If the credit cost of the data to be transmitted is less than the credits remaining on the receiving device, the data is transmitted. Otherwise, the transmitting device holds on to the data until the receiving device has sufficient credits free.

This method of flow control works well for normal loads on well-balanced, non-oversubscribed IB fabrics. However, if the fabric is unbalanced or oversubscribed or just heavily loaded, some links may be oversaturated with traffic beyond the ability of the credit mechanism to help.

Congestion can be observed by checking the IB error counters. When an IB device attempts to transmit data but the receiving device cannot receive data due to congestion, the PortXmitWait counter is incremented. If the congestion is so bad that the data cannot be transmitted before the time-to-live on the packet expires, the packet is discarded and the PortXmitDiscards counter is incremented. If you’re seeing high values of PortXmitWait and PortXmitDiscards counters, enabling congestion control may help manage congestion on your InfiniBand fabric.

How Does InfiniBand Congestion Control Work?

When an IB switch detects congestion on a link, it enables a special bit, called the Forward Explicit Congestion Notification (FECN) bit, which informs the destination device that congestion has been detected on the link. When the destination receives a packet marked with the FECN bit, the destination device notifies the sending device of the congestion via a Backwards Explicit Congestion Notification bit (BECN.)

When the source receives the BECN bit notification from the destination, the sending (source) device begins to throttle the amount of data it sends to the destination. The mechanism it uses is the credits system – by reducing the credits available to the destination, the size and rate of the packets are effectively decreased. The sending device may also add a delay between packets to provide the destination device time to catch up on data.

Over time, the source device increases credits for the destination device, gradually increasing the amount of packets sent. If the destination device continues to receive FECN packets from its switch, it again transmits BECN packets to the source device and the throttling is increased again. Without the reception of BECN packets from the destination device, the source device eventually returns to normal packet transmission. This balancing act is managed by congestion control parameters which require tuning for each environment.

After enabling InfiniBand congestion control and proper tuning, we realized a 15 percent improvement in our Panasas file system benchmark testing. PortXmitDiscards counters were completely clear, and PortXmitWait counters were significantly smaller, indicating that congestion control was doing its job.

Given that no additional hardware or other costs were required to achieve these results, a speed increase of 15 percent plus increased stability of the IB fabric was a nice result.

How Can I Enable InfiniBand Congestion Control?

Congestion control must be enabled on all IB devices and hosts, as well as on the IB subnet manager. This process includes turning on congestion control and setting a congestion control key on each device, as well as tuning the congestion control tables and parameters on each host and switch.

After congestion control is enabled on each IB device, the OpenSM configuration file must be modified to tune the subnet manager’s congestion control manager. Please note that mistuned parameters will either wreak havoc on a fabric or be completely ineffectual, so be careful – and do plenty of testing on a safe “test” system. Never attempt this on a live or production system.

Enabling InfiniBand congestion control had an immediate positive effect on our IB fabric. If you are suffering from issues with fabric congestion, enabling congestion control may provide the similar relief for your fabric as well, without the cost of adding additional hardware.

About the Author

Adam Dorsey is a systems administrator and site lead for RedLine Performance Solutions.

Subscribe to HPCwire's Weekly Update!

Be the most informed person in the room! Stay ahead of the tech trends with industy updates delivered to you every week!

ISC18’s Industrial Day Slate: Digital Twins, CFD for Automotive, HPC for SMEs

June 23, 2018

For enterprise IT strategists, this year’s Industrial Day (Tuesday, June 26) at ISC18 in Frankfurt will cover a range of topics – digital twins, AI and machine learning in automotive design, HPC for SME’s and deve Read more…

By Doug Black

What’s Hot and What’s Not at ISC 2018?

June 22, 2018

As the calendar rolls around to late June we see the ISC conference, held in Frankfurt (June 24th-28th), heave into view. With some of the pre-show announcements already starting to roll out, what do we think some of the Read more…

By Dairsie Latimer

Servers in Orbit, HPE Apollos Make 4,500 Trips Around Earth

June 22, 2018

The International Space Station shines a little brighter in the night sky thanks to what amounts to an orbiting supercomputer lofted to the outpost last year as part of a year-long experiment to determine if high-end com Read more…

By George Leopold

HPE Extreme Performance Solutions

HPC and AI Convergence is Accelerating New Levels of Intelligence

Data analytics is the most valuable tool in the digital marketplace – so much so that organizations are employing high performance computing (HPC) capabilities to rapidly collect, share, and analyze endless streams of data. Read more…

IBM Accelerated Insights

Taking the AI Training Wheels Off: From PoC to Production

Even though it seems simple now, there were a lot of skills to master in learning to ride a bike. From balancing on two wheels, and steering in a straight line, to going around corners and stopping before running over the dog, it took lots of practice to master these skills. Read more…

HPCwire Readers’ and Editors’ Choice Awards Turns 15

June 22, 2018

A hallmark of sustainability is this: If you are not serving a need effectively and efficiently you do not last. The HPCwire Readers’ and Editors’ Choice awards program has stood the test of time. Each year, our read Read more…

By Tiffany Trader

What’s Hot and What’s Not at ISC 2018?

June 22, 2018

As the calendar rolls around to late June we see the ISC conference, held in Frankfurt (June 24th-28th), heave into view. With some of the pre-show announcement Read more…

By Dairsie Latimer

Servers in Orbit, HPE Apollos Make 4,500 Trips Around Earth

June 22, 2018

The International Space Station shines a little brighter in the night sky thanks to what amounts to an orbiting supercomputer lofted to the outpost last year as Read more…

By George Leopold

HPCwire Readers’ and Editors’ Choice Awards Turns 15

June 22, 2018

A hallmark of sustainability is this: If you are not serving a need effectively and efficiently you do not last. The HPCwire Readers’ and Editors’ Choice aw Read more…

By Tiffany Trader

ISC 2018 Preview from @hpcnotes

June 21, 2018

Prepare for your social media feed to be saturated with #HPC, #ISC18, #Top500, etc. Prepare for your mainstream media to talk about supercomputers (in between t Read more…

By Andrew Jones

AMD’s EPYC Road to Redemption in Six Slides

June 21, 2018

A year ago AMD returned to the server market with its EPYC processor line. The earth didn’t tremble but folks took notice. People remember the Opteron fondly Read more…

By John Russell

European HPC Summit Week and PRACEdays 2018: Slaying Dragons and SHAPEing Futures One SME at a Time

June 20, 2018

The University of Ljubljana in Slovenia hosted the third annual EHPCSW18 and fifth annual PRACEdays18 events which opened May 29, 2018. The conference was chair Read more…

By Elizabeth Leake (STEM-Trek for HPCwire)

Cray Introduces All Flash Lustre Storage Solution Targeting HPC

June 19, 2018

Citing the rise of IOPS-intensive workflows and more affordable flash technology, Cray today introduced the L300F, a scalable all-flash storage solution whose p Read more…

By John Russell

Sandia to Take Delivery of World’s Largest Arm System

June 18, 2018

While the enterprise remains circumspect on prospects for Arm servers in the datacenter, the leadership HPC community is taking a bolder, brighter view of the x86 server CPU alternative. Amongst current and planned Arm HPC installations – i.e., the innovative Mont-Blanc project, led by Bull/Atos, the 'Isambard’ Cray XC50 going into the University of Bristol, and commitments from both Japan and France among others -- HPE is announcing that it will be supply the United States National Nuclear Security Administration (NNSA) with a 2.3 petaflops peak Arm-based system, named Astra. Read more…

By Tiffany Trader

MLPerf – Will New Machine Learning Benchmark Help Propel AI Forward?

May 2, 2018

Let the AI benchmarking wars begin. Today, a diverse group from academia and industry – Google, Baidu, Intel, AMD, Harvard, and Stanford among them – releas Read more…

By John Russell

How the Cloud Is Falling Short for HPC

March 15, 2018

The last couple of years have seen cloud computing gradually build some legitimacy within the HPC world, but still the HPC industry lies far behind enterprise I Read more…

By Chris Downing

US Plans $1.8 Billion Spend on DOE Exascale Supercomputing

April 11, 2018

On Monday, the United States Department of Energy announced its intention to procure up to three exascale supercomputers at a cost of up to $1.8 billion with th Read more…

By Tiffany Trader

Deep Learning at 15 PFlops Enables Training for Extreme Weather Identification at Scale

March 19, 2018

Petaflop per second deep learning training performance on the NERSC (National Energy Research Scientific Computing Center) Cori supercomputer has given climate Read more…

By Rob Farber

ORNL Summit Supercomputer Is Officially Here

June 8, 2018

Oak Ridge National Laboratory (ORNL) together with IBM and Nvidia celebrated the official unveiling of the Department of Energy (DOE) Summit supercomputer toda Read more…

By Tiffany Trader

Nvidia Responds to Google TPU Benchmarking

April 10, 2017

Nvidia highlights strengths of its newest GPU silicon in response to Google's report on the performance and energy advantages of its custom tensor processor. Read more…

By Tiffany Trader

Hennessy & Patterson: A New Golden Age for Computer Architecture

April 17, 2018

On Monday June 4, 2018, 2017 A.M. Turing Award Winners John L. Hennessy and David A. Patterson will deliver the Turing Lecture at the 45th International Sympo Read more…

By Staff

Google Chases Quantum Supremacy with 72-Qubit Processor

March 7, 2018

Google pulled ahead of the pack this week in the race toward "quantum supremacy," with the introduction of a new 72-qubit quantum processor called Bristlecone. Read more…

By Tiffany Trader

Leading Solution Providers

SC17 Booth Video Tours Playlist

Altair @ SC17

Altair

AMD @ SC17

AMD

ASRock Rack @ SC17

ASRock Rack

CEJN @ SC17

CEJN

DDN Storage @ SC17

DDN Storage

Huawei @ SC17

Huawei

IBM @ SC17

IBM

IBM Power Systems @ SC17

IBM Power Systems

Intel @ SC17

Intel

Lenovo @ SC17

Lenovo

Mellanox Technologies @ SC17

Mellanox Technologies

Microsoft @ SC17

Microsoft

Penguin Computing @ SC17

Penguin Computing

Pure Storage @ SC17

Pure Storage

Supericro @ SC17

Supericro

Tyan @ SC17

Tyan

Univa @ SC17

Univa

Google I/O 2018: AI Everywhere; TPU 3.0 Delivers 100+ Petaflops but Requires Liquid Cooling

May 9, 2018

All things AI dominated discussion at yesterday’s opening of Google’s I/O 2018 developers meeting covering much of Google's near-term product roadmap. The e Read more…

By John Russell

Pattern Computer – Startup Claims Breakthrough in ‘Pattern Discovery’ Technology

May 23, 2018

If it weren’t for the heavy-hitter technology team behind start-up Pattern Computer, which emerged from stealth today in a live-streamed event from San Franci Read more…

By John Russell

Nvidia Ups Hardware Game with 16-GPU DGX-2 Server and 18-Port NVSwitch

March 27, 2018

Nvidia unveiled a raft of new products from its annual technology conference in San Jose today, and despite not offering up a new chip architecture, there were still a few surprises in store for HPC hardware aficionados. Read more…

By Tiffany Trader

Sandia to Take Delivery of World’s Largest Arm System

June 18, 2018

While the enterprise remains circumspect on prospects for Arm servers in the datacenter, the leadership HPC community is taking a bolder, brighter view of the x86 server CPU alternative. Amongst current and planned Arm HPC installations – i.e., the innovative Mont-Blanc project, led by Bull/Atos, the 'Isambard’ Cray XC50 going into the University of Bristol, and commitments from both Japan and France among others -- HPE is announcing that it will be supply the United States National Nuclear Security Administration (NNSA) with a 2.3 petaflops peak Arm-based system, named Astra. Read more…

By Tiffany Trader

AMD’s EPYC Road to Redemption in Six Slides

June 21, 2018

A year ago AMD returned to the server market with its EPYC processor line. The earth didn’t tremble but folks took notice. People remember the Opteron fondly Read more…

By John Russell

Part One: Deep Dive into 2018 Trends in Life Sciences HPC

March 1, 2018

Life sciences is an interesting lens through which to see HPC. It is perhaps not an obvious choice, given life sciences’ relative newness as a heavy user of H Read more…

By John Russell

Intel Pledges First Commercial Nervana Product ‘Spring Crest’ in 2019

May 24, 2018

At its AI developer conference in San Francisco yesterday, Intel embraced a holistic approach to AI and showed off a broad AI portfolio that includes Xeon processors, Movidius technologies, FPGAs and Intel’s Nervana Neural Network Processors (NNPs), based on the technology it acquired in 2016. Read more…

By Tiffany Trader

Google Charts Two-Dimensional Quantum Course

April 26, 2018

Quantum error correction, essential for achieving universal fault-tolerant quantum computation, is one of the main challenges of the quantum computing field and it’s top of mind for Google’s John Martinis. At a presentation last week at the HPC User Forum in Tucson, Martinis, one of the world's foremost experts in quantum computing, emphasized... Read more…

By Tiffany Trader

  • arrow
  • Click Here for More Headlines
  • arrow
Do NOT follow this link or you will be banned from the site!
Share This