Building a Beowulf:My Perspective and Experience

Ron Choy

Lab. for Computer Science

MIT

Ver 1.02

First of all …

• Why do we care?– Make informed purchase decision– Know how to evaluate– Build one yourself!

Outline

• History/Introduction

• Hardware aspects

• Software aspects

• Our class Beowulf

• Beowulf design exercise

What is a Beowulf ?

• Massively parallel computer built out of COTS

• Runs a free operating system (not Wolfpack, MSCS)

• Connected by high speed interconnect

• Compute nodes are dedicated (not Network of Workstations)

Who uses Beowulfs?

• Pharmaceutical companies

• Investment firms

• Animation makers

• Me and you

The Beginning

• Thomas Sterling and Donald Becker CESDIS, Goddard Space Flight Center, Greenbelt, MD

• Summer 1994: built an experimental cluster

• Called their cluster Beowulf

The First Beowulf

• 16 x 486DX4, 100MHz processors

• 16MB of RAM each, 256MB in total

• Channel bonded Ethernet (2 x 10Mbps)

• Not that different from our Beowulf

The First Beowulf (2)

Current Beowulfs

• Faster processors, faster interconnect, but the idea remains the same

• Cluster database: http://clusters.top500.org/db/Query.php3

• Top cluster: 1.920 11.06 TFLOPS peak

Current Beowulfs (2)

Why Beowulf?

• It’s cheap!

• Our Beowulf, 18 processors, 9GB RAM: $15000

• A Sun Enterprise 250 Server, 2 processors, 2GB RAM: $16000

• Everything in a Beowulf is open-source and open standard - easier to manage/upgrade

Essential Components of a Beowulf

• Processors

• Memory

• Interconnect

• Software

Processors

• Major vendors: AMD, Intel

• AMD: Athlon MP

• Intel: Pentium 4

Comparisons

• Athlon MP and P4 are close in performance

• One is better for some applications and the other is better for other applications – it all depends on what you want

• However, Athlon MP is cheaper

Comparisons (2)

• P4 supports SSE2 instruction set, which perform SIMD operations on double precision data (2 x 64-bit)

• Athlon MP supports only SSE, for single precision data (4 x 32-bit)

Memory• DDR RAM (double data rate) – used

mainly by Athlons, P4 can use them as well

• RDRAM (Rambus DRAM) – used by P4s

Memory Bandwidth

• Good summary:

http://www6.tomshardware.com/mainboard/02q1/020311/sis645dx-03.html

• DDR beats out RDRAM in bandwidth, and is also cheaper

Interconnect

• The most important component

• Factors to consider– Bandwidth– Latency– Price– Software support

Ethernet

• Relatively inexpensive, reasonably fast and very popular.

• Developed by Bob Metcalfe and D.R. Boggs at Xerox PARC

• A variety of flavors (10Mbps, 100Mbps, 1Gbps)

Pictures of Ethernet Devices

                               

                               

Myrinet

• Developed by Myricom

• “OS bypass”, the network card talks directly to host processes

• Proprietary, but very popular because of its low latency and high bandwidth

• Usually used in high-end clusters

Myrinet pictures

Comparison

Fast Ethernet

Gigabit Ethernet

Myrinet

Latency ~120s ~120 s ~7 s

Bandwidth ~100Mbps peak

~1Gbps peak

~1.98Gbps real

Cost Comparison

• To equip our Beowulf with:– Fast ethernet: ~$1700– Gigabit ethernet: ~ $5600– Myrinet: ~$17300

How to choose?

• Depends on your application!

• Requires really low latency e.g. QCD? Myrinet

• Requires high bandwidth and can live with higher latency e.g. ScaLAPACK? Gigabit ethernet

• Embarrassingly parallel? Anything

What would you gain from fast interconnect?

• Our cluster: Single fast ethernet (100Mbps)– 36.8 GFLOPS peak, HPL: ~12 GFLOPS – 32.6% efficiency

• GALAXY: Gigabit ethernet– 20 GFLOPS peak, HPL: ~7 GFLOPS– 35% efficiency *old, slow tcp/ip stack!*

• HELICS: Myrinet 2000– 1.4 TFLOPS peak, HPL: ~864 GFLOPS– 61.7% efficiency

My experience with hardware

• How long did it take for me to assemble the 9 machines? 8 hours, nonstop

Real issue 1 - space

• Getting a Beowulf is great, but do you have the space to put it?

• Often space is at a premium, and Beowulf is not as dense as traditional supercomputers

• Rackmount? Extra cost! e.g. cabinet ~$1500, case for one node ~$400

Real issue 2 – heat management

• The nodes, with all the high powered processors and network cards, run hot

• Especially true for Athlons - can reach 60°C

• If not properly managed, the heat can cause crash or even hardware damage!

Real issue 3 - power

• Do you have enough power in your room?

• UPS? Surge protection?

• You don’t want a thunderstorm to fry your Beowulf!

• For our case we have a managed machine room - lucky

Real issue 4 - noise

• Beowulfs are loud. Really loud.

• You don’t want it on your desktop.

Bad idea

Noise (2)

• You might want to consider using ‘quiet’ power supplies and a diskless architecture

• Or a special purpose ‘personal cluster’

Real issue 5 - cables

• Color scheme your cables!

Software

• We’ll concentrate on the cluster management core

• Three choices:– Vanilla Linux/FreeBSD – Free cluster management software (a very

patched up Linux) – Commercial cluster management software (very

very patched up Linux, with technical support)

The issues

• Beowulfs can get very large (100’s of nodes)

• Compute nodes should setup themselves automatically

• Software updates must be automated across all the nodes

• Software coherency is an issue

Vanilla Linux

• Most customizable, easiest to make changes

• Easiest to patch

• Harder for someone else to inherit the cluster – a real issue

• Need to know a lot about Linux to properly setup

Free cluster management softwares

• Oscar: http://oscar.sourceforge.net/• Rocks: http://rocks.npaci.edu• MOSIX: http://www.mosix.org/• (usually patched) Linux that comes with software

for cluster management• Reduces dramatically the time needed to get

things up and running• Open source, but if something breaks, you have

one more piece of software to hack

Commercial cluster management

• Scyld: www.scyld.com - founded by Donald Becker

• Scyld – father of Beowulf

• Sells a heavily patched Linux distribution for clustering, free version available but old

• Based on bProc, which is similar to MOSIX

My experience/opinions

• I chose Rocks because I needed the Beowulf up fast, and it’s the first cluster management software I came across

• It was a breeze to setup• But now the pain begins … severe lack of

documentations• I have actually stripped the cluster of all

Rocks features now -> almost a plain RH7.1

Experience (2)

• Also a batch system like OpenPBS http://www.openpbs.org is a must in a multi-user environment

Software (cont’d)

• Note that I skipped a lot of details: e.g. file system choice (NFS? PVFS?), MPI choice (MPICH? LAM?), libraries to install …

• I could talk forever about Beowulfs but it won’t fit in one talk

Recipe we used for our Beowulf• Ingredients: $15000, 3 x 6 packs of coke, 1 grad

student1. Web surf for 1 weeks, try to focus on the

Beowulf sites, decide on hardware2. Spend 2 days filling in various forms for

purchasing and obtaining “competitive quotes”3. Wait 5 days for hardware to arrive, meanwhile

web surf some more, and enjoy the last few days of free time in a while

Recipe (cont’d)

4. Lock grad student, hardware (not money), and coke in an office. Ignore scream. The hardware should be ready after 8 hours.

Office of the future

Recipe (cont’d 2)

5. Move grad student and hardware to its final destination. By this time grad student will be emotionally attached to the hardware. This is normal. Have grad student set up software. This would take 2 weeks.

Our Beowulf

Things I would have done differently

• Plain Linux

• Color scheme the cables!

• Try a diskless setup (saves on cost and management – but no local scratch space)

• Get rackmount

Design a $30000 Beowulf

• One node (2 processors, 1GB RAM) costs $1400, with 4.6 GFLOPS peak

• Should we get:– 16 nodes, with fast ethernet, or– 8 nodes, with Myrinet?

Design (cont’d)

• 16 nodes with fast ethernet:– 73.6 GFLOPS peak

– 23.99 GFLOPS real (using the efficiency of our cluster)

– 16 GB of RAM

• 8 nodes with Myrinet– 36.8 GFLOPS peak

– 22.7 GFLOPS real (using the efficiency of HELICS)

– 8 GB of RAM

Design (cont’d 2)

• First choice is good if you work on embarrassingly parallel problems which does not require much communication

• Second choice is more general purpose