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Mohammed and the Mountain

Eric Jul

DIKUDepartment of Computer Science

University of Copenhagen

Mohammed and Mount Safa

IF THE MOUNTAIN WILL NOT COME TO MOHAMMED, MOHAMMED WILL GO TO THE MOUNTAIN

"If one cannot get one's own way, one must adjust to the inevitable. “

The legend goes that when the founder of Islam was asked to give proofs of his teaching, he ordered Mount Safa to come to him. When the mountain did not comply, Mohammed raised his hands toward heaven and said, 'God is merciful. Had it obeyed my words, it would have fallen on us to our destruction. I will therefore go to the mountain and thank God that he has had mercy on a stiff-necked generation.”

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Data and Computation

Question: where should computation go?

Near the source? Or near the destination?

Or somewhere in the middle?

Shall the mountain of data go to the destination?

OR

Shall the computation be moved toward the source?3

Source DestinationData

Problem Statement

At which points should the data be processed?

And how much? (Just filtering? Content-based?)

Close to destination:

• may be network-inefficient

Close to source:

• more complicated

• may require non-trivial cooperation in network

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Main Point:Spectrum: Moving Data or Computation

Spectrum ranging from moving all data to destination to moving all computation to the source

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Source DestinationData

Publisher Broker

Broker

Broker

Subscriber

Overview

• Mobility of Data and Computation

• Grid Computing Example

• Mobile Objects in Emerald

• Group Communication in Emerald

• Mobile Grid Applications – Evil Man

• Advice from experiences with dozens of Ph.D. projects

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Mobility of Data and Computation

What is computation?

Pub-sub systems: just filtering?

Flooding vs. Match-first

Flooding: filter at destination – simple, but network inefficient

Match-first: network efficient

Moving work closer to source as to reduce data transferred.

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Motivating Example with huge data sets

The Large Hadron Collider at CERN

ATLAS project

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LHC – Large Hadron Collider

Atlas project

All of LHC…

les robertson - cern-it-09-00 10 last update 2000-11-27 18:33

CERN

The LHC DetectorsCMS

ATLAS

LHCb

Raw recording rate 0.1 – 1 GB/sec3.5 PetaBytes / year

~108 events/year

Problem: Huge amounts of data

The problem is the large amount of data (Petabytes!) and the geographical distribution of the scientists wanting to look at / compute upon the data

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LHC Data Distribution

Data is collected at CERN & distributed to ~12 data centers.

Data is then distributed to sub-centers.

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CERNT1

T1T1

T1T1

T1T1T2

T1T1

T1T1

T1T1

T1T1

…

…

T2T2

T2T2

T2T2

…

Programs and Data Meet Up

Scientists can access the data and perform computation by submitting jobs to the sub-centers, e.g., via a Grid middleware

So both computation and data are moved – and meet at Grid centers

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Techniques to Consider

There are a number of general ideas/schemes to consider:

• Compressing Data

• Decoupling data and metadata, e.g., video & videoinfo

• Content Routing Protocol (Pascal’s talk)

• Caching

• Multicast

• Providing Mobile Objects and Mobile Computation

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Mobile Objects

Problem: efficient placement of data and computation in a distributed system

A solution: an Object-Oriented Language with:

– objects

– mobility of objects

– transparent remote calls

– group communication for pub-sub

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Mobile Objects in Emerald

Developed 1984-1988 at the University of WashingtonDistributed language and run-time support systemEmerald is an OO language:• “Pure” OO like Smalltalk – all data represented as objects (no

primitive types)• Algol-family syntax (statements are NOT objects)• Process concept (threads)• Synchronization (Hoare monitors)• Conformity based type system (worth several talks in itself)• Like Java, but simpler

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Emerald Spring Cleaning Collector

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Distribution Features

Concept of location: A node is merely a machine (within a semi-closed network)

• Mobility: move X to Y• Attachment allows groups to be moved• Location: loc <- locate X• “Remote” object invocation – transparent! X.f• Checkpoint: stable version to disk• Node failure: failure handler, unavailability• Immutable objects (instead of primitives)

Emerald Spring Cleaning Collector

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Example: Kilroy

const Kilroy == object Kilroy

process

var i: Integer <- 0

var myNode: Node <- locate self

var myList: Nodelist

var remoteNode: Node

myList <- myNode.getActiveNodes

for (i <- 0; i < myList.upperbound; i <- i+1)

remoteNode <- myList(i)$theNode

move Kilroy to remoteNode

end loop

end process

end Kilroy

Emerald Spring Cleaning Collector

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Example: Call By Move

const CallByMove == object CallByMove

process

var myData: LocalDataSet

var myHomeNode: Node <- locate self

var remoteNode: Node

X.f[move myData]

move self to X

X.f[myData]

move self to myHomeNode

end process

end Kilroy

Emerald Spring Cleaning Collector

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Example: Call By Visit

const CallByVisit == object CallByVisit

process

var myData: LocalDataSet

var myHomeNode: Node <- locate self

var remoteNode: Node

X.f[visit myData]

move self to X

X.f[myData]

move self to myHomeNode

end process

end Kilroy

Group Communication

Added group communication to Emerald

Introduce a group concept and a group multi-cast call

Calling a function on a group object calls all the group members

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Emerald Spring Cleaning Collector

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Group Communication

const GroupExample == object GroupExample

process

var subscriberGroup: Group.of[Subscriber]

subscriberGroup.Add[s1] ...

mySubscriberGroup <- subscribeGroup.getGroup

% using a group

mySubscriberGroup.notify[data]

end process

end GroupExample

New Problem Area

Switching to an entirely new problem area

Problem:

moving applications to Grid computing sites

Trust code?

Trust Operating System?

Proposed solution – named Evil Man

Move entire application including OS into a Grid Cluster

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OS Migration

Desire: move entire running applications

OS solution: pick up and move the ENTIRE OS

First, use a Virtual Machine Monitor (Zen, VMWare) to separate OS from hardware

Second, put migration code inside OS – it has all the needed functionality already – and get a Self-migrating OS

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Laundromat Computing with Evil Man

Evil Man is based on:

• Using virtual machines as containers for untrusted code• Using live VM migration to make execution independent of

location• Using micro-payments for pay-as-you-go computing

Evil Man uses Virtual Machine Migration to move actively executing applications into a Grid Cluster – e.g., to number-crush data from the Atlas project at CERN

Evil Man

Prototype cluster management system developed at the Danish Center for Grid Computing at the University of Copenhagen

One great Ph.D. student deserves most of the credit for the work:

Jacob Gorm Hansen

2008 Eurosys Rodger Needham award for Best Systems Ph.D.

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Virtual Machine Migration

• A VM is self-contained, including open files, sockets, shared memory

• Interface to virtual hardware is clearly defined – and simple

• All dependencies abstracted via fault-resilient network protocols

• VMs typically long-lived

process

file

process

VMM VMM

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Why Migration Downtime Matters

•Upsets users of interactive applications such as games

•May trigger failure detectors in a distributed system

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Live Migration Reduces Downtime

•The VM can still be used while it is migrating

•Data is transferred in the background, changes sent later

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Self-MigrationLetting the OS move itself

• Pre-copy migration relies on:– TCP/IP for transferring system state

– Paging for tracking of write accesses

• A VM is self-paging & has a TCP/IP stack

• Reduce VMM complexity by performing migration from within the VM

• No need for networking, threading or crypto in the TCB

VMVM VMVM VMVM

VMMVMM

Migration

Paging

TCP/IP

HardwareHardware

Paging

TCP/IP

Paging

TCP/IP

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An Inspiring Example of Self-Migration

von Münchhausen in the swamp

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First Iteration

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Delta Iteration

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Snapshot/Copy-on-Write Phase

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Dealing with Network Side-effects

• The copy-on-write phase results in a network fork

• “Parent” and “child” overlap and diverge

• Firewall network traffic during final copy phase

• All except migration-traffic is silently dropped in last phase

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Impact of Migration on Foreground Load

httperf

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Laundromat Computing

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Pay-as-you-go Processing

• Laundromats do this already– Accessible to anyone

– Pre-paid & pay-as-you-go

– Small initial investment

• We propose to manage clusters the same way– Micro-payment currency

– Pay from first packet

– Automatic garbage collection when payment runs out

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Token Payments

• Initial payment is enclosed in Boot Token

• Subsequent payments keep the VM alive as resources are consumed

• Use a simple hash-chain for recurring payments– Hn(s), Hn-1(s), …, H(s), s

• Boot Token signed by trusted broker service

• Broker handles authentication

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Injecting a New VM

• Two-stage boot loader handles different incoming formats– ELF loader for injecting a Linux kernel image

– Checkpoint loader for injecting a migrating VM

• “Evil Man” service decodes Boot Token “magic ping”

• Evil Man is 500 lines of code + network driver

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Laundromat Summary

• Pros:– Simple and flexible model

– Hundreds instead of millions LOC

– Built-in payment system

– Hypercube forks

– Self-scaling applications

• Cons:– Needs direct network access

– Network load

– Problems migrating across firewalls

Yet another switch

Some advice from my experience with a score of Ph.D. projects

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Things to think about

• State your problem

• Spread out initially, then focus

• Stay close to your focus

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State your problem

• Be able to answer the question: ”What is the problem you are trying to solve?” -- don’t end up with a ”solution in search of a problem!”

• State your problem

• Often good with a motivating example

• Often good with a narrow, less-general example that also can be used as a milestone – even a killer app

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Spread out initially, then focus

• Spread out, then narrow & focus (diagram)

• Establish a dissertation title when you start to focus

• Remember that your thesis focus is a SUBSET of all you have done – your dissertation is NOT a logbog of your work – nor a description of ”see what I have done” (diagram)

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Stay close to your focus

• Write a draft of your conclusion early – use it for focus

• Concentrate on your main contribution (diagram)

• Try for a good, narrow example that is implementable and can be a good demo and an excellent milestone

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Walk away with…

• Spectrum: moving data to computation site or moving computation to the data site

• Spectrum underlies many decisions concerning architecture of pub-sub systems

• Moving data is well-known; moving computation more tricky

• Don’t have a solution in search of a problem

• State the problem clearly – and state which part of it you will solve – use examples/killer apps

• As you go narrow your focus

• Shot for a good contribution73

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Questions?

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The End

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