Asynchronous Communication Mechanisms (ACMs) Fei Xia Fei Xia - - PowerPoint PPT Presentation

Asynchronous Communication Mechanisms (ACMs)

Fei Xia Fei Xia EECE, Newcastle University

• Point to point connection

writer reader data writer reader Data is a stream of items of a set type. Writer and reader are cyclic processes. Writer provides one item of data per cycle. Reader uses one item per cycle.

• Simple approach

reader writer

t

b a

• An ACM divides temporal domains

writer timing domain reader timing domain writer reader ACM temporal divide

• Each process updates its
• wn control variables
• A process can only read the

value of a control variable of another process

• Concurrent processes

ACM reader writer

Shared memory Control variables

data data

• The shared memory is organized

as a set of cells

• A cell is a memory location that

can store one data item

• Each process has its own set of

control variables

• Concurrent processes

executing on different timing domains

• 1-bit control variables
• Does not require any

synchronization primitives

• Order of execution does

not interfere on the correctness of the result

data data

• Atomic register [Lamport 86]
• The problem of asynchronous communication can be solved using

shared registers

• Complex registers are built from simple ones
• ACMs [Simpson 87-03]
• Defined a fully ACM that preserves data-coherence with 4 slots
• Defined a fully ACM that preserves data-coherence with 4 slots
• Classification of ACMs
• Systematic synthesis of ACMs [Yakovlev Group 96-08]
• ACMs are modelled using MATLAB for use in higher level systems
• Automatic synthesis of ACMs [Gorgônio, Xia 05-08]
• State space generation (BFS based)
• Petri net synthesis using theory of regions
• Modular approach using Petri nets modules as building blocks
• Modelling and verification using CPN

• BB

BB RRBB RRBB ! ! OWBB OWBB OWRRBB OWRRBB ! ! OWBB OWBB OWRRBB OWRRBB

• Overwriting enables the writer to be fully asynchronous
• Rereading enables the reader to be fully asynchronous
• BB stands for bounded buffer

• ACM

Reader Writer Control variables Shared memory data data variables enough memory to enough memory to contain an arbitrary contain an arbitrary number of items number of items

"

C C0 C C1

1

C C

• 1

1

C C2

2

• cells in a ring … …

cells in a ring … …

#$

• RRBB with three cells from an interleaving specification
• rereading

cyclic waiting

""

• Adding “silent actions”

µ µ

• λ
• λ

µ µ

• λ: writer silent actions;

µ: reader silent actions

µ

• µ

µ

• λ

µ

• λ

λ

• µ

µ

• µ

µ

• λ
• prepare to

reread

""

µ µ

• λ
• λ

µ µ

• var : 0..-1; : 0..-1; initialized sensibly

(say =-1) and initialize data items in the cells. writer wr: write cell ; ww: wait until ≠+1; w0: :=(+1 mod ); reader r0: if (+1 mod )≠ then :=(+1 mod ); rd: read cell ;

µ

• λ
• µ

µ

• λ

µ

• λ

λ

• µ

µ

• µ

µ

• λ
• w0: :=(+1 mod );

rd: read cell ;

""

Reader Writer cell memory and ACM cell cell memory data data cell memory and ACM cell +1 Reader Writer data data advance advance

• var : 0..1; : 0..1; initialized sensibly (one cell has =1 and

another has =1, all others being 0) and initialize data in the cells. %

• %
• wr: write;

r0: if next=0 then w0: :=0; next:=1; begin :=0; next:=1; ww: wait until next=0; advance to next end wa: advance to next; rd: read;

!

• Needs more than one “slot” per cell
• This will allow the writer to “overtake” the reader if overwriting the oldest

item in the buffer

• Item to be overwritten can be
• Oldest in the buffer
• Newest in the buffer
• Newest in the buffer
• Some other item (does not make as much sense as the previous choices)
• Also has to do with if the ACM is organized as a FIFO, a stack or a random

bag

&!%""

µ µ µ

• λ
• µ
• λ
• λ
• µ
• µ

µ

• λ
• µ

µ λ µ

• λ
• λ

λ µ µ

• µ

µ λ λ

&!%""

writer writer

Control variables

x x x

slot 0 cell 0 cell 1 cell 2

reader reader

One-hot enconding (1-bit variable)

Control variables data

x00 x10 x20

slot 0 slot 1

x01

!%""

var : 0..1; : 0..1; : 0..1 (this cell is overwritten); : 0..1 (writer releases cell for reader); %

• w0: next:=0;

r0: if =1 then w0: next:=0; r0: if =1 then if ≠ then :=1; begin :=0; =(not); advance to next end wr: write slot ; :=; wa: :=1; rd: read slot ; next:=0; ra: if next=1 then advance to next; advance to next

'

Alex Yakovlev Alex Yakovlev Hugo Simpson Ian Clark Eric Campbell Jordi Cortadella Kyller Gorgonio