Parallel Computer Architecture

606

CHAPTER

8 Directory-Based Cache Coherence

1.

P<i

sends read request

to

home node

for

A.

2.

P

2

sends read-exclusive request

to

home (for the write

of

A).

Home (serializer) won't process

it

until

it is

done

with read from P

1#

which

it

receives first.

3.

In

response

to

(1), home sends reply

to

P«|

(and sets

directory presence bit). Home now thinks read is

com-

plete (there are no acknowledgments

for

a read reply).

Unfortunately,

the

reply does

not

get

to

P-i right away.

4a.

In response

to

(2), home sends data reply

to

P

2

corre-

sponding

to

request

2.

4b.

In

response

to

(2), home sends invalidation

to

P-j;

it

reaches

P

1

before transaction

3

(n ...