🚀 go-pugleaf

In article <1992Sep11.121830.17069@hubcap.clemson.edu> zilla@nj.nec.com (John Lewis) writes:
>Someone told me that the SGI multiprocessor machines cannot run
>different unix processes on different processors; they can only
>run a single (but possibly parallelized) process at a given time.
>The reason was said to be that the machines do not implement
>any mechanism for simultaneous writes.
>
>Is this correct?  If so, how are simultaneous writes from
>different threads of a parallelized process prevented?

I don't know whether it's true or not.  But if it is, prevention of
simultaneous writes within a process can be prevented using a semaphore in
shared memory.  The write() routine would check this, and delay until the
semaphore is cleared.

>Is the new sun multiprocessor (4/690?) similar?

No, it's exactly opposite.  A process can only be on one processor at a
time, so each processor runs a different process.

Prevention of simultaneous writes is accomplished by only allowing one
processor to be in the kernel at a time.  Upcoming releases of the OS
removes this restriction, and uses locks around specific critical regions
of the kernel rather than the entire kernel.
-- 
Barry Margolin
System Manager, Thinking Machines Corp.

barmar@think.com          {uunet,harvard}!think!barmar