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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 592
SEC. 8.2
MULTICOMPUTERS
561
Replication
One improvement to the basic system that can improve performance consid-
erably is to replicate pages that are read only, for example, program text, read-only
constants, or other read-only data structures. For example, if page 10 in Fig. 8-22 is
a section of program text, its use by CPU 0 can result in a copy being sent to CPU
0 without the original in CPU 1’s memory being invalidated or disturbed, as shown
in Fig. 8-22(c). In this way, CPUs 0 and 1 can both reference page 10 as often as
needed without causing traps to fetch missing memory.
Another possibility is to replicate not only read-only pages, but also all pages.
As long as reads are being done, there is effectively no difference between replicat-
ing a read-only page and replicating a read-write page. However, if a replicated
page is suddenly modified, special action has to be taken to prevent having multi-
ple, inconsistent copies in existence. How inconsistency is prevented will be dis-
cussed in the following sections.
False Sharing
DSM systems are similar to multiprocessors in certain key ways. In both sys-
tems, when a nonlocal memory word is referenced, a chunk of memory containing
the word is fetched from its current location and put on the machine making the
reference (main memory or cache, respectively). An important design issue is how
big the chunk should be?
In multiprocessors, the cache block size is usually 32 or
64 bytes, to avoid tying up the bus with the transfer too long.
In DSM systems, the
unit has to be a multiple of the page size (because the MMU works with pages),
but it can be 1, 2, 4, or more pages.
In effect, doing this simulates a larger page
size.
There are advantages and disadvantages to a larger page size for DSM.
The
biggest advantage is that because the startup time for a network transfer is fairly
substantial, it does not really take much longer to transfer 4096 bytes than it does
to transfer 1024 bytes.
By transferring data in large units, when a large piece of
address space has to be moved, the number of transfers may often be reduced. This
property is especially important because many programs exhibit locality of refer-
ence, meaning that if a program has referenced one word on a page, it is likely to
reference other words on the same page in the immediate future.
On the other hand, the network will be tied up longer with a larger transfer,
blocking other faults caused by other processes. Also, too large an effective page
size introduces a new problem, called
false sharing
, illustrated in Fig. 8-23. Here
we have a page containing two unrelated shared variables,
A
and
B
.
Processor 1
makes heavy use of
A
, reading and writing it. Similarly, process 2 uses
B
frequent-
ly. Under these circumstances, the page containing both variables will constantly
be traveling back and forth between the two machines.
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