15 Dec
2023
15 Dec
'23
11:51 p.m.
For me, the term "system process" means either:
o A conventional, but perhaps privileged user-mode process that
performs a system function. An example would be the output side of a
spooling system, or an operator communications process.
o A process, or at least an address space + execution thread, that
runs in privileged mode on the hardware and whose address space is in
the resident kernel.
Do Unix system processes participate in time-sliced scheduling the way
that user processes do?
On 12/14/23, Bakul Shah <bakul(a)iitbombay.org> wrote:
Exactly! If blocking was not required, you can do the work in an
interrupt handler. If blocking is required, you can't just use the
stack of a random process (while in supervisor mode) unless you
are doing some work specifically on its behalf.
The usual programming convention for IBM S/360/370 operating systems
(OS/360, OS/VS, TOS and DOS/360, DOS/VS) did not involve use of a
stack at all, unless one was writing a routine involving recursive
calls, and that was rare. Addressing for both program and data was
done using a base register + offset. PL/I is the only IBM HLL I know
that explicitly supported recursion. I don't know how they
implemented automatic variables assigned to memory in recursive
routines. It might have been a linked list rather than a stack.
I remember when I first went from the IBM world and started
programming VAX/VMS, I thought it was really weird to burn an entire
register just for a process stack.
Interestingly, other early systems don't
seem to have thought of this
structuring technique.
I suspect IBM operating systems probably did use them. At least TSO
must have. Once you start *accounting* (and charging) for cpu time,
this idea must fall out naturally. You don't want to charge a process
for kernel time used for an unrelated work! There was a race condition in V7 swapping code. Once a
colleague and I
spent two weeks of 16 hour debugging days!
I had a race condition in some multithread code I wrote. I couldn't
find it the bug. I even resorted to getting machine code listings of
the whole program and marking the critical and non-critical sections
with green and red markers. I eventually threw all of the code out
and rewrite it from scratch. The second version didn't have the race
condition.
-Paul W.