QUESTION:
Our new SMP based systems include a nonblocking cross-bar switch.
The IBM salesmanual states the switch is capable of 800 MB/sec
sustained and 1.8 GB/sec peak. My customer wants to know under
what conditions this data was measured. What packet size did we use
for this test? Thanks.
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A: SMP systems.
The machine organization includes a data cross bar. The memory
port is 288 bits wide (32 bytes) and can support 800 MB/sec. The
data ports (one per pair of processors) are 72 bits wide (8 bytes)
and can provide 266.67 MB/sec.
So, the actual memory bandwidth available may be limited by either
the number of memory banks or number of processors. Clearly, we
need four memory banks and four processors to sustain 800 MB/sec.
Addresses can be issued every three clock cycles and up to four
independent memory accesses can be in progress at any given time
(pipelined). Memory READs may thus complete with three cycle
periodicity. 32 bytes is transferred from/to the crossbar for
every memory operation. After any possible ECC correction, the
data is routed inside the crossbar to the appropriate processor port.
Processor ports are 72 bits wide (eight bytes) and four clock
cycles are used to transfer the 32 byte data block. Thus, there
is ALWAYS a one cycle overlap within the crossbar whenever there
are two adjacent memory READs. This provides an instantaneous
bandwidth of 1200 MB/sec, even though the actual delivery rate
from the memory is 800 MB/sec. Additionally, there can be a cache
to cache transfer occurring between processors on the other two
data ports, so there may be another data transfer occurring on
the same physical cycle described above. Thus, the peak
instantaneous rate is 3x600 MB/sec (the peak transfer rates of
three crossbar ports).
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This item was created from library item Q676285 BBBJD
Additional search words:
BAR BBBJD CROSS HARDWARE IX JAN95 MEASURED NEW NONBLOCKING OZIBM
OZNEW RATE RISCSYSTEM RISCSYSU SMP SPEED SWITCH SWITCHING SYS SYSTEM
UNIT
WWQA: ITEM: RTA000052610 ITEM: RTA000052610
Dated: 10/1996 Category: RISCSYSU
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