Which Problems Does a Multi-Language Virtual Machine Need to Solve in the Multicore/Manycore Era?

Oct 26, 20110 likes503 views

While parallel programming for very regular problems has been used in the scientific community by non-computer-scientists successfully for a few decades now, concurrent programming and solving irregular problems remains hard. Furthermore, we shift from few expert system programmers mastering concurrency for a constrained set of problems to mainstream application developers being required to master concurrency for a wide variety of problems. Consequently, high-level language virtual machine (VM) research faces interesting questions. What are processor design changes that have an impact on the abstractions provided by VMs to provide platform independence? How can application programmers’ diverse needs be facilitated to solve concurrent programming problems? We argue that VMs will need to be ready for a wide range of different concurrency models that allow solving concurrency problems with appropriate abstractions. Furthermore, they need to abstract from heterogeneous processor architectures, varying performance characteristics, need to account for memory access cost and inter-core communication mechanisms but should only expose the minimal useful set of notions like locality, explicit communication, and adaptable scheduling to maintain their abstracting nature. Eventually, language designers need to be enabled to guarantee properties like encapsulation, scheduling guarantees, and immutability also when an interaction between different problem-specific concurrency abstractions is required.

Which
Problems
Does
a

Mul$-‐Language
Virtual
Machine

Need
to
Solve
in
the

Mul$core/Manycore
Era?

Stefan
Marr

Ma8as
De
Wael,
Michael
Haupt,
Theo
D’Hondt

VMIL
Workshop
2011

2011-‐10-‐24

VMs
today

Performance
Memory
Management

F#

MulJ-‐Language
Eco
System
PlaLorm
Independent

25/10/11
2

What
do
we
need
in
the

MulJcore/Manycore
Era?

25/10/11
3

Two
Dimensions

Processor
Design
Concurrency
Models

Actors

STM

DataFlow

Fork/Join

25/10/11
4

Processors
and
Concurrency
Models

VM?

!&'()*

!&'()*
!"#$%!&'()*

6$4&)78()&%9&4,():
!&'()*

!&'()*
!"#$%!&'()*

25/10/11
+&,%-()&. !"$*%-()&. 5

./")&0%'&'()* 1")#(2.%0#33&)&$4%0&.#5$.

WHAT
ARE
THE
RELEVANT
CHANGES
AT

THE
HARDWARE-‐LEVEL?

25/10/11
6

Brawny
vs.
Wimpy

•  High
sequenJal
•  Weak
sequenJal

performance
performance

•  Few
in
numbers
•  High
in
numbers

25/10/11
7

And
everything
in-‐between!

Brawny
Hybrid
Wimpy

IBM
Power7
Intel
MIC

Intel
i7

AMD
Fusion
Cell
B.E.

Nvidia
Fermi

25/10/11
8

Cores
vs.
Threads
vs.
Shared
PUs

Source:
AMD

25/10/11
9

Memory
and
CommunicaJon

Nehalem
Cache
Hierarchy
Four
Processor
Setup

CommunicaJon
implicit
via
memory
Source:
Intel

25/10/11
10

Memory
and
CommunicaJon

TILEPro64

Source:
Tilera

25/10/11
11

What
do
VMs
need
to
account
for?

•  Appropriate
uJlizaJon

–  Varying
performance
of
cores/units?

–  Caches
and
simultaneous
mul,threading?

•  Non-‐uniform
memory
and
communicaJon

–  Cache
hierarchies?

–  Explicit
inter-‐core
communicaJon?

25/10/11
12

Actors

STM

DataFlow

Fork/Join

WHAT
ARE
TYPICAL
IMPLEMENTATION

PROBLEMS
OF
CONCURRENCY

MODELS?

25/10/11
13

EncapsulaJon

IMPORTANT:
Messages
can

actor
BreakEncapsulation:
be
any
kind
of
object
but

have
to
be
immutable.
Scala

def
main_loop(self):
can’t
enforce
immutability

receive:
(yet)
so
this
has
to
be
by

convenJon.

(Message)
=>

Message.msg
=
"broken"
JCSP:
obj-‐by-‐ref,
no
warning

#
main
Kilim:
zero-‐copy
type
system

msg
=
Message("Hello
World!")

not
“reﬂecJon-‐proof”

broken
!
(msg)
Clojure:
STM
+
pure
Java

Agents
+
pure
Java

#
wait

print
msg.msg
Swing
UI:
“The
single-‐

thread
rule”

25/10/11
14

Scheduling
Guarantees

Fork/Join
Actors
PrioriJzed
Tasks

work-‐stealing
fair
scheduling
priority-‐based
scheduling

25/10/11
15

Immutability

Source:
Star
Wars
Episode
V:
The
Empire
Strikes
Back.

25/10/11
16

$Crossing Borders and Language-‐Levels jThread.sortedSet = FarRefWrapper(sortSet) e a jThread.sortedSet .add(FarRefWrapper(e)) e.compareTo(*) add(e) // add(fr_e) (wrapper semantics) fr_e.compareTo(obj) { return send_wait(e, compareTo, obj) sortedSet } Language/ApplicaJon-‐Level ImplementaJon/Meta-‐Level 25/10/11 17$

What
do
VMs
need
to
account
for?

•  Enforcing
language
semanJcs?

•  Flexible
concurrent
encapsulaJon?

–  What
is
the
relaJon
to
immutability?

•  Enforceable
scheduling
guarantees?

•  Normal
vs.
reﬂecJve
operaJons?

25/10/11
18

What
do
VMs
need
to
account
for?

Processor
Design
Concurrency
Models

Actors

STM

DataFlow

Fork/Join

25/10/11
19

What
do
VMs
need
to
account
for?

Processor
Design
Concurrency
Models

•  Appropriate
uJlizaJon
•  Flexible
concurrent

–  Varying
performance?
encapsulaJon?

–  Hyperthreads?
–  Flexible
Immutability?

•  Non-‐uniform
memory
and
•  Enforceable
scheduling

communicaJon
guarantees?

–  Cache
hierarchies?
•  Normal
vs.
reﬂecJve

–  Explicit
inter-‐core
operaJons?

communicaJon?

25/10/11
20

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The multicore era comes with new challenges for software engineers. The Software Languages Lab and its Parallel Programming Group does research on languages and language runtimes/virtual machines for the new era of multi- and manycore hardware. This talk gives a brief introduction to the problem and names our approaches to tackle the challenges. However, since this is presented as a Pecha Kucha, the talk is very abstract and focuses on conveying the theme instead of technical details.

Locality and Encapsulation: A Foundation for Concurrency Support in Multi-Lan...Stefan Marr

We propose to search for common abstractions for different concurrency models to enable high-level language virtual machines to support a wide range of different concurrency models. This would enable domain-specific solutions for the concurrency problem. Furthermore, advanced knowledge about concurrency in the VM model will most likely lead to better implementation opportunities on top of the different upcoming many-core architectures. The idea is to investigate the concepts of encapsulation and locality to this end. Thus, we are going to experiment with different language abstractions for concurrency on top of a virtual machine, which supports encapsulation and locality, to see how language designers could benefit, and how virtual machines could optimize programs using these concepts.

Insertion Tree Phasers: Efficient and Scalable Barrier Synchronization for Fi...Stefan Marr

This paper presents an algorithm and a data structure for scalable dynamic synchronization in fine-grained parallelism. The algorithm supports the full generality of phasers with dynamic, two-phase, and point-to-point synchronization. It retains the scalability of classical tree barriers, but provides unbounded dynamicity by employing a tailor-made insertion tree data structure. It is the first completely documented implementation strategy for a scalable phaser synchronization construct. Our evaluation shows that it can be used as a drop-in replacement for classic barriers without harming performance, despite its additional complexity and potential for performance optimizations. Furthermore, our approach overcomes performance and scalability limitations which have been present in other phaser proposals.

Encapsulation and Locality: A Foundation for Concurrency Support in Multi-Lan...Stefan Marr

Intermediate Language Design of High-level Language VMs: Towards Comprehensiv...Stefan Marr

The document discusses intermediate language design for virtual machines to provide comprehensive concurrency support. It surveys 17 existing virtual machines to evaluate their concurrency support and models. Only 6 provided explicit concurrency support in the intermediate language. Support ranged from low-level instructions to high-level constructs in standard libraries. To enable new concurrency concepts, future virtual machines need intermediate languages that support both low-level and high-level concurrency features, with an open question around the tradeoffs of features in the intermediate language versus the standard library.

Virtual Machine Support for Many-Core Architectures: Decoupling Abstract from...Stefan Marr

The upcoming many-core architectures require software developers to exploit concurrency to utilize available computational power. We argue that today's virtual machines (VMs), which are a cornerstone of software development, do not provide sufficient abstraction for concurrency concepts. To overcome this shortcoming, we propose to integrate concurrency operations into VM instruction sets. Since there will always be VMs optimized for special purposes, our goal is to develop a methodology to design instruction sets with concurrency support. Therefore, we also propose a list of tradeoffs that have to be investigated to advise the design of such instruction sets. As a first experiment, we implemented one instruction set extension for shared memory and one for non-shared memory concurrency. From our experimental results, we derived a list of requirements for a full-grown experimental environment for further research.

VMADL: An Architecture Definition Language for Variability and Composition ...Stefan Marr

High-level language virtual machines (VMs) can be used on a wide range of devices as a basic part of the deployed software stack. As the available devices differ to a large degree in their applications and their available resources, distinct implementation strategies have to be used for certain parts of a VM to meet the special requirements. This paper motivates the need for an architecture definition language for complex software systems like VM implementations. The basic concepts and language constructs of this language, which is called VMADL, are introduced. To motivate further discussions, the benefits of this approach are briefly discussed.