Introduction to Software Distributed

Shared Memory Systems

Chang-Yi Lin2004 / 02 / 26

Outlines What is a software DSM system? Message-passing vs. Shared-memory How does it work? Memory Consistency Models Cache Coherence Implementation Levels Granularity

What is a software DSM system? A distributed-memory system (often called a multicomputer) consist of multiple independent processing nodes with local memory modules, connected by a general interconnection network. Global shared memory.

What is a software DSM system? A DSM system logically implements the

shared-memory model on a physically distributed-memory system.

The DSM system hides the remote communication mechanism from the application writer, preserving the programming ease and portability typical of shared-memory systems.

Message-passing vs. Shared-memory Two different programming type. Shared-memory programming is easier.

Message-passing vs. Shared-memory Message-passing

Point-to-point communication

Message-passing vs. Shared-memory Message-passing

buffers and data types blocking and nonblocking

Message-passing vs. Shared-memory

Message-passing vs. Shared-memory Pthread

Thread 1

LockA = A++;unlock

Thread 2

LockA = A++;unlock

Thread 3

LockA = A++;unlock

How does it work?

DSM system

Memory

Shared-memory applications

Interconnection network

How does it work?

Interconnection network

Mem MemMemMem

DSM DSM DSM DSM

App AppAppApp

How does it work?

Memory Consistency Models What is Memory consistency?

在單機上 P1: w(x)1 R(x)1---------------------------------- time

在兩台機器上 P1: w(x)1 P2: R(x)?---------------------------------------- time

Memory Consistency Models A consistency model is essentially a

contract between the software and the memory.

If the software agrees to obey certain rules, the memory promises to work correctly.

If the software violates these rules, correctness of memory operation is no longer guaranteed.

Memory Consistency Models Strict consistency Sequential consistency (SC) Release consistency (RC) Scope consistency (ScC)

Memory Consistency Models Strict consistency

Definition: any read to memory location x returns the value stored by the most recent write operation to x.

Impossible to DSM

P1: w(x)1P2: R(x)1---------------------------------------- time

Memory Consistency Models Sequential consistency (SC)

Definition: the result of any execution is the same as if the operations of all processors were executed in some sequential order, and the operations of each individual processor appear in this sequence in the order specified by its program.

Any valid interleaving is acceptable behavior, but all processes must see the same sequence of memory reference.

Memory Consistency Models Sequential consistency (SC)

P1(A) a=1;(B) Print(b,c);

P2(C) b=1;(D) Print(a,c);

P3(E) c=1;(F) Print(a,b);

(A)(B)(C)(D)(E)(F)

(A)(C)(D)(B)(E)(F)

(C)(E)(F)(D)(A)(B)

(A)(C)(E)(B)(D)(F)

Memory Consistency Models Release Consistency (RC)

Two types of access Ordinary access: read and write Synchronization access: acquire lock, release lock

and barrier Rules:

Before an ordinary access to a shared variable is performed, all previous acquires done by the process must have completed successfully.

Before a release is allowed to be performed, all previous reads and writes done by the process must have completed.

Memory Consistency Models Release Consistency (RC)

Rule1: …Acq(L1)…Rel(L1) … Acq(L5) …Acq(L3)…R(x)Rule2: Acq(L2) w(x) R(y) R(z) Rel(L2)

Example:P1: Acq(L) w(x)1 w(x)2 Rel(L)P2: Acq(L) R(x)2 Rel(L)P3: R(x)?

Memory Consistency Models Scope consistency (ScC)

Memory Consistency Models Relaxing consistency permits temporary inconsistencies (delayed updates)

Lazy release consistency (LRC) (TreadMarks, CVM) Scope consistency (ScC) (JIAJIA, JUMP)

Cache Coherence Write invalidate

Suffer from false sharing Write update

Too expansive when many replicas Work best in application with tight sharing

Implementation Levels Modifying OS kernel

IVY (SC): modifying the memory management unit (MMU) of OS to map between the shared virtual memory address space and the local memory. Language level

Linda, Orca User-level runtime library

Trademarks, CVM, JIAJIA, JUMP, Brazos Combination of multiple implementation levels, even hardware support

Munin, Midway, NCP2

Granularity The choice of the block size depends on

the cost of communcation 1 byte message v.s. 1024 byte message

Locality of reference in the application Most DSM systems use a page-based granularity with 1K byte to 8K byte. Larger page size, better locality of reference

系統 開發者 實作層次 顆粒度 一致性模型 一致性協定IVY Yale 函式庫＋作業系統 頁 (1KB) SC WI

Munin Rice 函式庫＋作業系統 可變 Eager RC WU/WI

TreadMarks Rice 函式庫 頁 (4KB) LRC MW ， WICVM Maryland 函式庫 頁 LRC-MW ， LRC-SW ，SC WU

Midway CMU 函式庫＋編譯器 可變 EC ， PC ， RC WU

NCP2 UFRJ, Brail 函式庫＋硬體支援 頁 (4KB) EC ， RC WU/WI

Quarks Utah 函式庫 region 、頁 RC ， SC WU/WI ， MWsoftFLASH Standford 作業系統 頁 (16KB) RC ， DIRC FLASH-like

Cashmere-2L Rochester 函式庫 頁 (8KB) HLRC WU

Brazos Rice 函式庫 頁 ScC Early update ，WUShasta DEC WRL 編譯器 可變 SC WI

Mermaid Toronto 函式庫＋作業系統 頁 (1KB ， 8KB) SC WI

Dsoftware DSM6K IBM Research 作業系統 頁 (4KB) SC WI

Mirage UCLA 作業系統 512Bytes SC WI

JIAJIA 中國科學院 函式庫 頁 (4KB) ScC WI

Simple-COMA SICS(Sweden) and SUN 作業系統 頁 SC WI

Blizzard-S Wisconsin 函式庫 快取行 SC WI

Shrimp Princeton 作業系統＋硬體支援 頁 AURC ， SC WU/WI

Linda Yale 語言 可變 SC Impl.dependentOrca Vrije Univ., Netherlands 語言 可變 同步相關 WU