Offloading vs. Onloading: The Case of CPU Utilization

By Gilad Shainer, Mellanox

June 18, 2016

One of the primary conversations these days in the field of networking is whether it is better to onload network functions onto the CPU or better to offload these functions to the interconnect hardware.

Onloading interconnect technology is easier to build, but the issue becomes the CPU utilization; because the CPU must manage and execute network operations, it has less availability for applications, which is its primary purpose.

Offloading, on the other hand, seeks to overcome performance bottlenecks in the CPU by performing the network functions, as well as complex communications operations, such as collective operations or data aggregation operations, on the data while it moves within the cluster. Data is so distributed these days that a performance bottleneck is created by waiting for data to reach the CPU for analysis. Instead, data can be manipulated wherever it is located within the network by using intelligent network devices that offload functions from the CPU. This has the added advantage of increasing the availability of the CPU for compute functions, improving the overall efficiency of the system.

The issue of CPU utilization is one of the primary points of contention between the two options. How you measure CPU utilization and what type of benchmark you use for the test can provide highly misleading results.

For example, a common mistake is to use a common latency test or message rate test to determine the CPU utilization; however these tests typically require the CPU to constantly look for data (that is, polling data on the memory), which makes it seem as though the CPU is at 100 percent utilization, when actually it is not working at all. Using such a test to determine CPU utilization will produce a false result. In the real world, CPUs do not constantly check for data.

So what is the proper way to measure CPU utilization? Ideally, a data bandwidth test or another test that does not use data polling can be used to determine CPU utilization. Alternatively, if a message rate test is used, the test must be configured to avoid data polling loops in order to produce realistic results. Ultimately, the best option is to compare the number of CPU instructions that were actually executed against the number of CPU instructions that could possibly have been executed during the duration of the test. This produces an accurate percentage of CPU utilization.

Another important element to consider is the type of overhead that is being measured. For example, if the test is designed to measure the impact of the network protocol on CPU utilization, the test should only test data transfers between two servers, and not include additional overheads such as MPI, which is in the software layer. If the purpose is to measure the overhead of a software framework, such as MPI, an MPI test should be used, but in that case, the proper MPIs with the proper offloads must be used, if they exist. Not all MPIs support various hardware-based offloads, so it is important to beware of the test conditions.

So now that it’s clear how to measure CPU utilization accurately, the question remains: Which is better, offloading or onloading? We have conducted multiple data throughput tests between servers connected with EDR InfiniBand and the proprietary Omni-Path alternative.

The tests included send-receive data transfers at the maximum data speed supported by each interconnect (~100Gb/s) while measuring the CPU utilization (Table 1). At the data speed of 100Gb/s, InfiniBand only consumed 0.8 percent CPU utilization, while Omni-Path required 59 percent CPU utilization for the same task. Therefore, the CPU availability for the application in the InfiniBand case is 99.2 percent, while for Omni-Path, only 40.4 percent of the CPU cycles are available for applications. Furthermore, we have measured the CPU frequency in each of the cases, since the CPU can reduce its frequency to save power when it is not required to perform at full speed. For the InfiniBand case, the CPU frequency was able to drop to 59 percent of is nominal frequency to enable power saving. For the Omni-Path case, on the other hand, the CPU was performing at full speed, so no power saving could be achieved.

CPU Utilization Comparison

Table 1 – CPU Utilization Comparison

The tool that was used to review the CPU stats was the Intel Performance Counter Monitor toolset. The tool provides a richer set of measurements that provide a detailed system status. Utilizing this tool, we found that Omni-Path did not actually reach the 100G speed, but fell a little short at 95Gb/s. The AFREQ stats reported the CPU frequency that was dynamically set during the test. We were also able to view the number of iterations and active cycles used per the different interconnect protocols (Table 2).

Intel Performance Counter Monitor Tool stats

Table 2 – Intel Performance Counter Monitor Tool stats

Moreover, when InfiniBand is implemented on intelligent devices within the Co-Design architecture, it can further reduce overhead on the CPU by offloading MPI operations as well. Of course, to measure this, the test must be sure to include the software layer in the benchmark such that an accurate real-world result is received. We plan to perform various further tests at different applications levels in the future to demonstrate the significant advantages of InfiniBand.

Ultimately, InfiniBand implements offloading specifically in order to reduce the overhead on the CPU, and, as the testing herein indicates, it works exactly as it was designed. If someone shows results that indicate otherwise, it is worthwhile to investigate the circumstances of the testing to better understand how the results were achieved. In all likelihood, the results are misleading and do not accurately reflect real-world conditions.

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!

New Panasas High Performance Storage Straddles Commercial-Traditional HPC

November 13, 2018

High performance storage vendor Panasas has launched a new version of its ActiveStor product line this morning featuring what the company said is the industry’s first plug-and-play, portable parallel file system that d Read more…

By Doug Black

SC18 Student Cluster Competition – Revealing the Field

November 13, 2018

It’s November again and we’re almost ready for the kick-off of one of the greatest computer sports events in the world – the SC Student Cluster Competition. This is the twelfth time that teams of university undergr Read more…

By Dan Olds

US Leads Supercomputing with #1, #2 Systems & Petascale Arm

November 12, 2018

The 31st Supercomputing Conference (SC) - commemorating 30 years since the first Supercomputing in 1988 - kicked off in Dallas yesterday, taking over the Kay Bailey Hutchison Convention Center and much of the surrounding Read more…

By Tiffany Trader

HPE Extreme Performance Solutions

AI Can Be Scary. But Choosing the Wrong Partners Can Be Mortifying!

As you continue to dive deeper into AI, you will discover it is more than just deep learning. AI is an extremely complex set of machine learning, deep learning, reinforcement, and analytics algorithms with varying compute, storage, memory, and communications needs. Read more…

IBM Accelerated Insights

New Data Management Techniques for Intelligent Simulations

The trend in high performance supercomputer design has evolved – from providing maximum compute capability for complex scalable science applications, to capacity computing utilizing efficient, cost-effective computing power for solving a small number of large problems or a large number of small problems. Read more…

OpenACC Talks Up Summit and Community Momentum at SC18

November 12, 2018

OpenACC – the directives-based parallel programing model for optimizing applications on heterogeneous architectures – is showcasing user traction and HPC impact at SC18. Most noteworthy is that five of 13 CAAR applic Read more…

By John Russell

New Panasas High Performance Storage Straddles Commercial-Traditional HPC

November 13, 2018

High performance storage vendor Panasas has launched a new version of its ActiveStor product line this morning featuring what the company said is the industry Read more…

By Doug Black

SC18 Student Cluster Competition – Revealing the Field

November 13, 2018

It’s November again and we’re almost ready for the kick-off of one of the greatest computer sports events in the world – the SC Student Cluster Competitio Read more…

By Dan Olds

US Leads Supercomputing with #1, #2 Systems & Petascale Arm

November 12, 2018

The 31st Supercomputing Conference (SC) - commemorating 30 years since the first Supercomputing in 1988 - kicked off in Dallas yesterday, taking over the Kay Ba Read more…

By Tiffany Trader

OpenACC Talks Up Summit and Community Momentum at SC18

November 12, 2018

OpenACC – the directives-based parallel programing model for optimizing applications on heterogeneous architectures – is showcasing user traction and HPC im Read more…

By John Russell

How ASCI Revolutionized the World of High-Performance Computing and Advanced Modeling and Simulation

November 9, 2018

The 1993 Supercomputing Conference was held in Portland, Oregon. That conference and it’s show floor provided a good snapshot of the uncertainty that U.S. supercomputing was facing in the early 1990s. Many of the companies exhibiting that year would soon be gone, either bankrupt or acquired by somebody else. Read more…

By Alex R. Larzelere

At SC18: GM, Boeing, Deere, BP Talk Enterprise HPC Strategies

November 9, 2018

SC18 in Dallas (Nov.11-16) will feature an impressive series of sessions focused on the enterprise HPC deployments at some of the largest industrial companies: Read more…

By Doug Black

SC 30th Anniversary Perennials 1988-2018

November 8, 2018

Many conferences try, fewer succeed. Thirty years ago, no one knew if the first SC would also be the last. Thirty years later, we know it’s the biggest annual Read more…

By Doug Black & Tiffany Trader

CEA’s Pick of ThunderX2-based Atos System Boosts Arm

November 8, 2018

Europe’s bet on Arm took another step forward today with selection of an Atos BullSequana X1310 system by CEA’s (French Alternative Energies and Atomic Ener Read more…

By John Russell

Cray Unveils Shasta, Lands NERSC-9 Contract

October 30, 2018

Cray revealed today the details of its next-gen supercomputing architecture, Shasta, selected to be the next flagship system at NERSC. We've known of the code-name "Shasta" since the Argonne slice of the CORAL project was announced in 2015 and although the details of that plan have changed considerably, Cray didn't slow down its timeline for Shasta. Read more…

By Tiffany Trader

TACC Wins Next NSF-funded Major Supercomputer

July 30, 2018

The Texas Advanced Computing Center (TACC) has won the next NSF-funded big supercomputer beating out rivals including the National Center for Supercomputing Ap Read more…

By John Russell

IBM at Hot Chips: What’s Next for Power

August 23, 2018

With processor, memory and networking technologies all racing to fill in for an ailing Moore’s law, the era of the heterogeneous datacenter is well underway, Read more…

By Tiffany Trader

Requiem for a Phi: Knights Landing Discontinued

July 25, 2018

On Monday, Intel made public its end of life strategy for the Knights Landing "KNL" Phi product set. The announcement makes official what has already been wide Read more…

By Tiffany Trader

CERN Project Sees Orders-of-Magnitude Speedup with AI Approach

August 14, 2018

An award-winning effort at CERN has demonstrated potential to significantly change how the physics based modeling and simulation communities view machine learni Read more…

By Rob Farber

House Passes $1.275B National Quantum Initiative

September 17, 2018

Last Thursday the U.S. House of Representatives passed the National Quantum Initiative Act (NQIA) intended to accelerate quantum computing research and developm Read more…

By John Russell

Summit Supercomputer is Already Making its Mark on Science

September 20, 2018

Summit, now the fastest supercomputer in the world, is quickly making its mark in science – five of the six finalists just announced for the prestigious 2018 Read more…

By John Russell

New Deep Learning Algorithm Solves Rubik’s Cube

July 25, 2018

Solving (and attempting to solve) Rubik’s Cube has delighted millions of puzzle lovers since 1974 when the cube was invented by Hungarian sculptor and archite Read more…

By John Russell

Leading Solution Providers

TACC’s ‘Frontera’ Supercomputer Expands Horizon for Extreme-Scale Science

August 29, 2018

The National Science Foundation and the Texas Advanced Computing Center announced today that a new system, called Frontera, will overtake Stampede 2 as the fast Read more…

By Tiffany Trader

US Leads Supercomputing with #1, #2 Systems & Petascale Arm

November 12, 2018

The 31st Supercomputing Conference (SC) - commemorating 30 years since the first Supercomputing in 1988 - kicked off in Dallas yesterday, taking over the Kay Ba Read more…

By Tiffany Trader

HPE No. 1, IBM Surges, in ‘Bucking Bronco’ High Performance Server Market

September 27, 2018

Riding healthy U.S. and global economies, strong demand for AI-capable hardware and other tailwind trends, the high performance computing server market jumped 28 percent in the second quarter 2018 to $3.7 billion, up from $2.9 billion for the same period last year, according to industry analyst firm Hyperion Research. Read more…

By Doug Black

Intel Announces Cooper Lake, Advances AI Strategy

August 9, 2018

Intel's chief datacenter exec Navin Shenoy kicked off the company's Data-Centric Innovation Summit Wednesday, the day-long program devoted to Intel's datacenter Read more…

By Tiffany Trader

Germany Celebrates Launch of Two Fastest Supercomputers

September 26, 2018

The new high-performance computer SuperMUC-NG at the Leibniz Supercomputing Center (LRZ) in Garching is the fastest computer in Germany and one of the fastest i Read more…

By Tiffany Trader

Houston to Field Massive, ‘Geophysically Configured’ Cloud Supercomputer

October 11, 2018

Based on some news stories out today, one might get the impression that the next system to crack number one on the Top500 would be an industrial oil and gas mon Read more…

By Tiffany Trader

D-Wave Breaks New Ground in Quantum Simulation

July 16, 2018

Last Friday D-Wave scientists and colleagues published work in Science which they say represents the first fulfillment of Richard Feynman’s 1982 notion that Read more…

By John Russell

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

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