gpudet: a deterministic gpu architecture

Hadi Jooybar GPUDet: A Deterministic GPU Architecture 1

GPUDet: A Deterministic GPU Architecture

Hadi Jooybar1, Wilson Fung1, Mike O’Connor2, Joseph Devietti3, Tor M. Aamodt1

1The University of British Columbia2AMD Research3University of Washington


• GPUs are …• Fast• Energy efficient• Commodity hardware

But…

× Mostly use for certain range of applications

Why?

Communication among concurrent threads 1000s of Threads


0 __global__ void BFS_step_kernel(...) {1 if( active[tid] ) {2 active[tid] = false;3 visited[tid] = true;4 foreach (int id = neighbour_nodes){5 if( visited[id] == false ){6 cost[id] = cost[tid] + 1;7 active[id] = true;8 *over = true;9 } } } }

V0

V2V1

Cost = -Active = -

Cost = -Active = -

V0

V2V1

Cost = 1Active = 1

Cost = 1Active = 1

V0

V2V1

Cost = 1Active = 1

Cost = 2Active = 1

Motivation

BFS algorithmPublished in HiPC 2007


I will debug it this time

What about debuggers?!

The bug may appear occasionally or in different places in each run.

OMG! Where was that bug?!

Motivation


GPUDetStrong Determinism (hardware proposal)

Same Outputs Same Execution Path

Makes the program easier to Debug Test


0 __global__ void BFS_step_kernel(...) {1 if( active[tid] ) {2 active[tid] = false;3 visited[tid] = true;4 foreach (int id = neighbour_nodes){5 if( visited[id] == false ){6 cost[id] = cost[tid] + 1;7 active[id] = true;8 *over = true;9 } } } }

V0

V2V1

Cost = 1Active = 1

Cost = 2Active = 1

Motivation

BFS algorithmPublished in HiPC 2007


GPUDetStrong Determinism

Same Outputs Same Execution Path

Makes the program easier to Debug Test

×There is no free lunch× Performance OverheadOur goal is to provide Deterministic

Execution on GPU architectures with acceptable performance overhead


DRAMGPU Architecture

Compute Unit

Memory Unit

L1 Cache

ALUALUALU

DRAML2 Cache

Workgroups

CPUKernel launch

workgroup 2workgroup 1workgroup 0

x = input[threadID];y= func(x);output[threadID] = y;


Outline

• Introduction• GPU Architecture• Challenges • Deterministic Execution with GPUDet• GPUDet Optimizations

• Workgroup-Aware Quantum Formation• Deterministic parallel commit using Z-Buffer Unit• Compute Unit level serialization

• Results and Conclusion


Normal Execution

T0

T1

T2

T3

Deterministic GPU Execution Challenges

• Isolation mechanism• Provide method to pause execution of a thread

…Quantum 0

T0

T1

T2

T3

Quantum n

T0

T1

T2

T3

…Isolation

T0

T1

T2

T3

Communication Isolation

T0

T1

T2

T3

Communication


…


• Isolation mechanism• Lack of private caches • Lack of cache coherency

• Provide method to pause execution of a thread• Single Instruction Multiple Threads (SIMT)• Potential deadlock condition• Major changes in control flow hardware• Performance overhead workgroupn

wavefront



• Very large number of threads• Expensive global synchronization• Expensive serialization

• Different program properties• Large number of short running threads• Frequent workgroup synchronization• Less locality in intra thread memory accesses


Outline





if (tid < 16) x[tid%2] = tid;

x[0] = 0

T0

Coalescing Unit

x[1] = 1

T1

x[0] = 2

T2

x[1] = 15

T15

Deterministic Execution of a Wavefront

Data RaceMask v v - - - - - - … -

Address x

Data 14 15 - - - - - - … -

x[0] = 14 x[1] = 15 Not modifiedTo memory

…

Execution of one wavefront is deterministic



• Isolation mechanism• Provide method to pause execution of a thread

…Isolation

T0

T1

T2

T3

Communication Isolation

T0

T1

T2

T3

Communication

wavefront granularity

not a challenge anymore


Reaching Quantum Boundary

Global Memory

Read Only

Store Buffers

Local Memory

Wavefronts

…

Load Op CommitAtomic Op

• GPUDet-Basic

1. Instruction Count2. Atomic Operations3. Memory Fences4. Workgroup Barriers5. Execution Complete


Outline





Workgroup-Aware Quantum Formation• Extra global synchronizations

Load Imbalance

Reducing number of synchronizationsAvoid unnecessary quantum termination


AES

BFSr

BFSf

CFD C

P

HO

TSP

LIB

LPS

SRA

D HT

ATM

CLop

t0%

20%

40%

60%

80%

100% Atomic OperationsInstruction CountExecution CompleteWorkgroup Barriers

%of

Ter

min

ation

Rea

sons

Workgroup-Aware Quantum Formation

Quanta are finished by workgroup barriers

All reach a workgroup barrier

Continue execution in the parallel mode

Workgroup-Aware Decision Making


AES

BFSr

BFSf

CFD C

P

HO

TSP

LIB

LPS

SRA

D HT

ATM

CLop

t0%

20%

40%

60%

80%

100% Atomic OperationsInstruction CountExecution CompleteWorkgroup Barriers

%of

Ter

min

ation

Rea

sons

Finish execution of the Kernel function

Workgroup-Aware Decision Making

Workgroup-Aware Quantum Formation

Deterministic workgroup partitioning


Deterministic Parallel Commit using the Z-Buffer Unit

∞ ∞ ∞ ∞ ∞ ∞∞ ∞ ∞ ∞ ∞ ∞∞ ∞ ∞ ∞ ∞ ∞∞ ∞ ∞ ∞ ∞ ∞∞ ∞ ∞ ∞ ∞ ∞

∞ ∞ ∞ ∞ ∞ ∞∞ ∞ ∞ ∞ ∞ ∞7 7 7 ∞ ∞ ∞7 7 7 ∞ ∞ ∞7 7 7 ∞ ∞ ∞

8 8 8 8 8 88 8 8 8 8 87 7 7 8 8 87 7 7 8 8 87 7 7 8 8 8

8 8 5 5 8 88 8 5 5 5 87 5 5 5 5 57 5 5 5 5 55 5 5 5 5 5

Depth Buffer

Store Buffer Contents ≈ Color Values

Wavefront ID ≈ Depth Values

Z-Buffer Unit


• GPUs preserve Point to Point Ordering

AAA

AAA

Serialization is only among compute units

Compute Unit Level Serialization


Outline





ResultsAE

S

BFSr

BFSf

CFD C

P

HO

TSP

LIB

LPS

SRA

D HT

ATM

CLop

t00.5

11.5

22.5

33.5

44.5

5

Serial Mode

Commit Mode

Parallel Mode

Nor

mal

ized

Ex

ecuti

on T

ime

2x Slowdown

• GPGPU-Sim 3.0.2Applications with atomic operations


20% Performance Improvement for application with barriers

19% Performance Improvement for application with small kernel functions

Quantum FormationAE

S

BFSr

BFSf

CFD C

P H LIB

LPS

SRA

D HT

ATM

CLop

t

AVG

0

1

2

3

4

5

GPUDet-baseWorkgroup BarrierEnd of the Kernel

Nor

mal

ized

Exec

ution

Tim

e


Deterministic Parallel Commit using the Z-Buffer UnitZ-

Buffe

r

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

Z-Bu

ffer

Lock

ing

AES BFSr BFSf CFD CP HOTSP LIB LPS SRAD HT ATM Clopt

0

2

4

6

8

10#REF! #REF!

Nor

mal

ized

Exe

cutio

n Ti

me

60% Performance Improvement on Average


Compute Unit Level Serialization

W-S

er

CU-S

er

W-S

er

CU-S

er

W-S

er

CU-S

er

CLopt HT ATM

02468

101214

Serial Mode Series2Series1

Nor

mal

ize

Exec

ution

Tim

e6.1x Performance Improvement in

Serial Mode


Conclusion

• Encourages programmers to use GPUs in broader

range of applications• Exploits GPU characteristics to reduce performance

overhead• Deterministic execution within a wavefront• Workgroup-aware quantum formation• Deterministic parallel commit using Z-Buffer Unit• Compute Unit level serialization

Questions?

gpudet: a deterministic gpu architecture

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