Stephen Finch/SSI1 Stephen_Finch/SSI1.SSI1 at
Tue May 23 04:21:04 PDT 1995

As we continue to increase SCSI Parallel speeds, we must begin to look at the
overhead associated with arbitration, selection and phase changes.  Back in
the good old days when these were just a small portion of the time spent on 
a SCSI command, these timings were considered insignificant.  But today, 

A 4096 byte transfer being transferred using FAST-40 and wide bus lasts for 
or 51.2 microseconds.  The processing of the command is:  arbitrate, select, 3 
of message out (identify, queue tag), command out (10 bytes), message in 
bus free, arbitrate, reselect, 3 bytes of message in, data, status in, message 
in, bus free.
(This is a read, but a write is the about the same with only the data transfer 
being moved,
or (worse) an additional reselection to transfer the data.)

Let's total the time (assume async transfers of msg/cmd/sts are a 5 
 Phase  ns
 -------  --------
 Arb  2400
 Selection 1600
 msg out  1000
 cmd out  2400
 msg in  1000
 bus free 0

 Arb  2400
 Reselection 1600
 msg in  1000
 data    51200
 sts in  1000
 msg in  1000
 bus free 0
 ----------- --------  -------
   15400  51200
15.4 out of 66.6 is a 23% overhead for a 4096 byte transfer!  

When SCSI first came out (5 megabytes/sec), the overhead was 1.8%.
With Fast/narrow the overhead was 3.6%.  
With Fast/wide or Fast-20 narrow:  7%.
With Fast-20/wide or Fast-40 narrow, the overhead becomes 13%.  
With Fast-40/wide would be 23%.
With Fast-40/32bits would be 38%.

The overhead is, of course, a function of the block length.  Assuming a single 
transfer of the data at full bus speeds using FAST-40/wide:

 Size  Overhead
 --------  ---------
 512  71%
 1024  55%
 2048  37%
 4096  23%
 8192  13%
 16384  7%
 32768  3.6%
 65536  1.8%
 131072  .9%
 262144  .47%
 524288  .23%
 1048576  .12%

The questions are:  
 Are we only interested in large transfers?
  Or, are transfers less than 16K of interest?  
 What is the acceptable overhead level?

When looking at Fast-40, I think it is time to also look at the overhead!


steve.finch at

Silicon Systems Inc.

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