Grid Computing: Overview and Research Issues

Grid ComputingGrid ComputingOverview and Research IssuesOverview and Research Issues

Peter Kelly

Adelaide University, Australia

[email protected]

Supervisors: Paul CoddingtonAndrew Wendelborn

What is grid computing?What is grid computing?

Grid computing is many things to many people

At its core, it’s about Sharing computing resources between

organisations Enabling more complex and demanding

applications by providing widespread access to powerful computers and storage

Integrating existing systems together

What is grid computing?What is grid computing?

In some respects it’s similar to cluster computing, however each computer may

Be located in a different country Use a different CPU architecture Run a different operating system Be owned by a different organisation Have a different amount of memory, disk space, and

computing power, and network bandwidth Not be available all of the time

Thus grids are much more complex than clusters!

Why is it useful?Why is it useful?

Demand for computing power is growing rapidly– In industry, science, government, engineering,

entertainment, defence… everywhere Need ways to harness the large amount of

computing power available around the world Organisations often want to collaborate on

projects and share resources with each other Grids provide the infrastructure to integrate

different applications that need to collaborate with each other to get useful work done

Types of grid computingTypes of grid computing

Service Oriented Architecture (SOA) Job submission (supercomputer access) Cycle stealing

Service Oriented Architecture Service Oriented Architecture (SOA)(SOA)

Applications are exposed as services, which provide a well-defined interface and are accessed through standard protocols

Clients use remote procedure calls to access these services

Client Service

Request

Response

Benefits of SOABenefits of SOA

SOA is platform agnostic– Client doesn’t need to know how service is implemented– Service doesn’t need to know how client is implemented

SOA is vendor independent– Based on open standards – no “lock in”– All SOA vendors support the same standards to enable

interoperability

SOA is widely supported– Many companies are getting behind it– Being adopted widely in commercial and scientific

organisations

Job submissionJob submission

Many organisations have large supercomputers (SMP or clusters) that they want users to be able to submit jobs to

This can be achieved by installing middleware on each supercomputer which interfaces to the local job queue– e.g. Globus GRAM - allows users to submit to job queues

such as PBS, LSF, etc. Users submit jobs to a superscheduler which manages a

“higher level” queue and dispatches jobs to resources The grid middleware handles tasks such as copying files to

and from the execution node, monitoring job progress, and abstracts the details of these away from clients

Job submissionJob submission

SMP machineCluster Cluster

Superscheduler

Client Client Client

Benefits of Job Submission GridsBenefits of Job Submission Grids

Users do not have to worry about differences between job submission systems running on different resources

Superschedulers make it possible to automatically find resources that will execute the job quicker

A user submits a job to a grid, it runs, and they get the results back later

Job submission can be implemented on top of SOA by providing a service with methods for submitting and monitoring jobs, as well as notifying clients of failures or completion– e.g. Globus MMJFS – provides a web service interface to

allow users to submit jobs

Cycle stealingCycle stealing

The use of large numbers of desktop PCs to run “embarrassingly parallel” applications

A master node coordinates execution and hands out tasks to workers

The worker process on each machine polls the master for work to do, and then executes the tasks as they become ready

Worker detects when the machine is being used by a user and suspends/aborts the active task

This model is inherently fault tolerant; if a machine dies or a task is aborted it can just be sent to another worker

Cycle stealingCycle stealing

Worker

Master

Worker Worker Worker

Benefits of cycle stealingBenefits of cycle stealing

Organisations can use their existing infrastructure to run computationally demanding applications– No need to invest in large SMP systems or clusters

Large-scale internet projects can get free computing power– …provided they can convince users to donate CPU time– e.g. SETI@Home

Cheap supercomputing Generally easy to deploy

So what So what reallyreally is grid computing? is grid computing?

Not really one specific technology or concept More of an umbrella term, like “networking” or “operating

system” Any (concrete) discussion of grid computing requires all

parties involved to agree on a definition of what features they are focusing on

Very much dependent upon what you want to do – different types of organisations have different requirements

Sometimes the lines are blurred and numerous systems support multiple “types” of grid computing

Lots of hype – can be very confusing at first!– it took me about a year to understand it enough to be able

to figure out what I wanted to do in my project

Web servicesWeb services

Web services are a particular type of SOA Based on standards from W3C and others:

– WSDL - language for defining service interfaces– SOAP - format used for exchange of messages– UDDI - directory mechanism for locating services– XML - used as standard encoding mechanism used by

WS protocols– … and many more

Web services are supported by all major programming languages– either as part of built-in APIs or add-on libraries

Today web services are the most popular mechanism for integrating systems together in and between organisations

Web service compositionWeb service composition

A programming model based on composing together functionality provided by multiple web services

Similar to the use of shared libraries/DLL files– common functionality provided by shared entity (service)– composition program builds additional functionality by

making use of one or more services Service composition programs can themselves be exposed

as web services– Can then be accessed by clients– Or used as part of even higher-level service compositions

Most popular language at present is BPEL (Business Process Execution Language)

SOA programming vs. remote SOA programming vs. remote executionexecution

Web services allow you to invoke programs already installed on a remote machine

Remote code execution allows you to execute arbitrary code on a remote machine

The latter is used for job submission and cycle stealing systems

Our research investigates a combination of these approaches– Provides ability to invoke and expose web services– Provides a distributed execution environment

Execution EnvironmentsExecution Environments

Problem: Need a standard way of executing arbitrary code remotely

SOA doesn’t give you this– it only standardises the protocols for different applications

to interact with each other Job submission systems don’t give you this

– only standardise the means of submitting and monitoring jobs – but not how they are actually executed

Cycle stealing requires this– existing cycle stealing systems these days typically specify

Java or .NET, or use app-specific worker code– but there is no standard which allows us to do this on an

internet scale

What is an execution environmentWhat is an execution environment

Instruction set– e.g. x86, PPC, SPARC, Java bytecode, .NET bytecode

API library– e.g. WIN32, POSIX, Java class libraries, .NET class

libraries Applications are always compiled for a specific execution

environment Can have different implementations of that environment

– x86 - AMD, Intel– Java - Sun, IBM, various open source efforts– .NET - Microsoft, Mono project

Applications compiled for a particular execution environment can run on any implementation of it

Virtual machinesVirtual machines

Common way of implementing an execution environment Abstracts away from underlying hardware/OS, providing

platform independence In a grid containing machines of different CPU architectures

and operating systems this is necessary to provide seamless access

To enable code to be executed anywhere, each machine on the grid must provide the same execution environment

Currently popular virtual machines:– Java Virtual Machine (JVM)– .NET Common Language Runtime (CLR)

A grid execution environment?A grid execution environment?

Problem: No standard execution environment supported by the popular grid middleware

Standardisation efforts (GGF) to date have focused only service interfaces, not implementation

Each grid middleware system provides its own set of APIs, and is targeted at different VMs/OSs

Applications are not yet portable between different middleware systems– At least not in the same sense that bytecode-compiled

code is portable– Compatibility exists only at the service interface level

Standardisation?Standardisation?

My belief: We won’t see the full potential of grid computing until we

have agreement on a standard execution environment Currently only SOA aspects are standardised

– But this goes only half way to solving the problem

This is is very much an open research issue Obvious candidates are Java and .NET

– But are they sufficient? Should they be extended? What about other alternatives?

– Much research already done into VM technology– But not so much in the grid community– IMHO a very important issue! More research needed here

Standardisation?Standardisation?

It’s just like the web Early web pages were static, as there was no support for

executing code in the browser; code only ran server-side– In the grid world this corresponds to SOA

Then came early versions of JavaScript/DHTML– Lack of standardisation, browsers were incompatible

Now we have a standard, widely supported, platform independent execution environment on 300+ million computers worldwide (JavaScript/ECMAScript)– And look what happened… client side web apps, AJAX,

Google maps, “Web 2.0” and the rest I predict grid will go through the same evolution

Our current researchOur current research

Investigating how to combine SOA and remote code execution programming models

Development of a new virtual machine + language implementation targeted at grid applications

GridXSLT An implementation of the XSLT programming

language– Supports web service composition– Executes programs across a grid in parallel– Provides a natural way to deal with XML data

Why XSLT?Why XSLT?

Ideal for manipulating XML data Has a “semantic match” with many properties of web

services Is a functional language and can be automatically

parallelised W3C standard with a sizable existing user base

– We wanted to avoid the challenges of trying to design a new language and introduce it to the world

– Better to just develop a new implementation of an existing one which is already popular

Support for XML dataSupport for XML data

XSLT is specifically designed for dealing with XML data All web services exchange data in XML format Java, C#, C++ etc. are less suitable for manipulating XML

because they are not designed for this (and in fact pre-date XML)– XML data is a “second class citizen” in these languages

and must be accessed through library functions or converted into objects

– APIs like DOM, SAX, etc. are less intuitive than built-in language constructs

– Conversion to objects carries significant overheads and risks losing information (e.g. element ordering)

We argue that XSLT is therefore a useful approach to developing composite web services

Pass by value semanticsPass by value semantics

Another mismatch between OO languages and web services is the way in which function arguments are handled

OO languages use pass by reference semantics - allowing a function to modify its arguments and the caller to see those changes

Web services use pass by value - where a new copy of each argument is made and a function can only transfer information to its caller through the return value

When using an OO language for WS development, the programmer must be aware of this and it can sometimes lead to mistakes

As a side effect-free functional language, XSLT uses pass by value, avoiding this problem

Parallel executionParallel execution

XSLT is a functional language Functions and loops do not have side effects - there

is no global state that can be modified This enables automatic parallelisation

– All arguments to a function call can be evaluated in parallel

– All iterations of a loop can be evaluated in parallel The programmer never needs to even know that

their program will be run in parallel– No dealing with threads, synchronisation, critical sections,

message passing, race conditions etc…– The underlying runtime system deals with all these issues

Implementing XSLTImplementing XSLT

We use a technique called graph reduction, a common way if implementing functional languages

A program is represented as a graph Execution proceeds by performing a series of

transformations on the graph

Graph reduction: ExampleGraph reduction: Example

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4 2


2*(3+4) @

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4 2


2*7 @

@ 7

* 2


2*7 @

@ 7

* 2


14 14

Parallel graph reductionParallel graph reduction

Graph reduction permits the possibility of parallel execution by allowing multiple parts of the graph to be reduced in parallel

Each processor in a parallel computer or cluster can manipulate a separate portion of the graph


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Functional programming for grids?Functional programming for grids?

It permits Automatic, seamless parallelism Automatic, seamless fault tolerance Automatic, seamless distribution

But… Some programs are based on state, which is in conflict with

the pure functional programming model– Although there are ways to get around this, e.g. monads

Different programming style to what most people are used to– Involves a learning curve– But might be worth it to get the above benefits– …depending on your needs

SummarySummary

Grid computing is a very diverse area– Many different types of systems– Many different requirements– Useful in many areas

Different “types” of grid computing– SOA, job submission, cycle stealing– Others as well that I haven’t discussed here

Lots of challenges and open research questions– e.g. defining a suitable execution environment for grid

applications– This is just one of many!

SummarySummary

Our research project - GridXSLT An attempt to combine different grid

computing models– SOA– Remote code execution/cycle stealing

Aims to make the programmer’s job easier– Parallelisation handled by the compiler– Suited to dealing with XML data exchanged by

web services and stored in XML databases– High-level language which hides underlying

details

Websites of interestWebsites of interest

Global Grid Forum– http://www.ggf.org

Grid Café (introduction to grid computing)– http://www.gridcafe.org

IBM - grid computing– http://www.ibm.com/grid

GridXSLT– http://gridxslt.sourceforge.net

Updates on my research– http://pmkelly.blogspot.com

Documents

Grid Computing: Overview and Research Issues