False sharing – IBM HANDBOOK 260 User Manual
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RS/6000 43P 7043 Models 150 and 260 Handbook
M (modified)
The addressed sector is valid in this cache only. The value in
this sector has been changed in the cache, but the change is
not yet reflected in memory.
E (exclusive)
The addressed sector is valid in this cache only. The data is
consistent with system memory.
S (shared)
The addressed sector is valid in this cache and at least one
other cache. It is still consistent with system memory.
I (invalid)
The addressed sector is not valid in the cache.
2.5.1.8 False Sharing
Figure 17 illustrates false sharing. Here, a whole cache unit of storage (a
cache sector in the case of the IBM SMP systems) is represented.
Figure 17. False Sharing
Suppose processor 1 and processor 2 loaded the same memory address in
their cache. If processor 1 changes only a portion of the cache sector, d1 for
example, the cache consistency logic will invalidate all the sectors in the
processor 2 cache. Then, if processor 2 tries to modify another portion of its
cache sector, d2, for example, which is still invalid since the whole sector is
invalid, a cache miss will occur. This is called false sharing.
Processor 1
Processor 2
Memory
Cache:
d1
d2
...
Cache:
d1
d2
...