PSF/AIX use of multiple processor RS/6000 models and
ITEM: RTA000107553
Q:
ABSTRACT: PSF/AIX use of multiple processor RS/6000 models and
resultant performance implications.
SEARCH ARG: psf aix multiple processors
TOPIC THREAD: PRINT
PSF/AIX
..
How does PSF/AIX exploit RS/6000 multiprocessor models like the F50?
If a dedicated print server has 4 processors installed, what benefits
will be achieved:
1. Four processor RS/6000 driving one IP4000 printer?
Is PSF/AIX capable of using two or more processors in the server,
assuming a single processor does not have the power to
handle the load and keep the printer running at rated speed?
2. Four processor RS/6000 driving four IP4000 printers?
Is PSF/AIX capable of running the workload to drive each of the
four printers on one of the unique processors at the same time?
(Either explicitly or effectively)
--------------------------------------------------------------
(In other words, how does the the threading design of PSF/AIX within
the AIX operating system environment allow for the effective
utilization of 4-way RS/6000 servers in the above scenarios?
A:
Let me append a previous ASKQ item that may answer your questions.
If that information is not sufficient, please reopen the item and I
will consult with Boulder.
Closing for now, but please reopen if needed.
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TITLE: Can Process id functions be set up to run of specific
processors in a 7043-240 SMP Environment.
TOPIC: IX RISCSYSTEM_SOFTWARE OP_SYS OP_SYS_OTHER
******* This item has not been reviewed and may not be complete. ******
QUESTION:
My customer would like to know how workload is balanced on a 7043-240.
They are interested in using a 7043-240 with 256MB of memory and 18GB
of disk to receive and print some large files coming from a Mainframe
via the MVS Download function of PSF. They want to know if they can
dedicate the input manager function of PSF on one processor so that
input manager can receive and segment the print file and the "print
driver" function of PSF which will print segmented files on athe other
processor.
Can the customer set up the Operating System to perform this function
or is it automatically performed?
RESPONSE:
First let me say that although PSF for AIX does not explicitly
exploit an SMP system, the performance analysts in Boulder have seen
cases where PSF/AIX does benefit from running on an SMP system. This
is attributed to the balancing that the MP version of the operating
system does to assign processes to an idle CPU. So even without
explicit assignment of PSF processes to a specific CPU, a customer
may see performance benefits from running on an SMP system.
Having said that, it is possible to assign a process and its threads
to a specific CPU in an SMP system. The command is called
"bindprocessor". The PSF/AIX performance analyst I consulted
referenced the redbook "Understanding IBM RS/6000 Performance and
Sizing", SG24-4810-00, page 269. This is an excellent book for
performance information on the RS/6000. The command "bindprocessor"
is available as an AIX command and is not a developer-only function
call. The command syntax can be found in Volume 1 of the AIX 4.1
command reference.
He did tell me that Boulder has not tried this command with PSF/AIX.
It may be that AIX itself can do a better job of assigning tasks to
idle processors for workload balancing than a casual user who could
inadvertently cause himself bottlenecks. In general, the analyst
did say that he doesn't think that it's a good idea for a user to
to bind specific processes to a single processor. However, he/she can
go ahead and try - it may reduce the number of cache misses which
is the advantage of doing it.
I'd also make sure you know in advance how to undo this in the event
it creates more problems than it solves. Also, if the customer tries
this and later calls in for support, particularly for performance
issues, it would be very important for the Support Center to know
that this binding had been done.
--------------------------------------------------------------------
Q:
The key question revolves around the statement that the customer *may*
see performance benefits running on an SMP. From my perspective, if
the customer is buying 4 processors, I have to have a better idea of
the value of those 4 processors than they "may" see some benefit.
Like, "Four processors will be equivalent to 2.5 X the spec rating of
each processor, etc." Since I don't know how many processes/threads
are running in PSF/AIX while driving 4 printers I can speculate on this.
Thanks for your insights.
A:
I spoke with the PSF/AIX performance analyst and received the following
information.
PSF/AIX is not multithreaded. It can and does, however, use multiple
processes. For example, there is a "primary" or converter process
that converts the incoming data to IPDS; then there is a "secondary"
process that drives the IPDS to the printer. So for a single printer,
in an SMP system, PSF will be using a minimum of two processes that
AIX can dispatch to separate CPUs.
In addition, there are generally additional processes running. For
example, there could be processes to handle I/O, spooling, transforms,
segmenting, or general AIX operating system processes. So in an SMP
with four or more CPUs, additional tasks could be dispatched
simultaneously.
Regarding your specific questions:
R1) For a single IP4000 on a 4-way processor, PSF will use two of the
processors (with AIX doing the dispatching automatically) for the
primary and secondary processes. In general (although this can vary
inversely), we've seen the primary process use 90% of one CPU and
the secondary process use about 10% of the other CPU. If a CPU is
idle, then AIX MP code will dispatch other processes automatically.
It is possible with very intense print jobs for one of the processes
to "max out" the CPU on which it's running. If that happens, the
additional cycles needed cannot be assigned to another available
processor, and you will thus hit a bottleneck.
On the other hand, if you were using a uni-processor model with the
same workload, you'd hit the bottleneck sooner because you're having
to run all the required processes on a single CPU rather than
spreading them across multiple CPUs.
R2) "Is PSF/AIX capable of running the workload to drive each of the
four IP4000 printers on one of the unique processors at the same
time?"
You can technically use the bindprocessor command discussed earlier
to bind a process to a specific processor, but you really don't want
to do that. It is much better to let AIX handle the dispatching of
workload to idle CPUs. If you hard-tie a process to a certain CPU,
then you will force it to wait until that CPU becomes available even
though other CPUs in the system are idle. You're much more likely
to see performance problems and clutching/back-hitching if you were
to try this. Boulder definitely recommends that you do NOT bind PSF
processes to a specific CPU on an SMP system; let the AIX operating
system handle the dispatching of processes for you. That will be
your most effective use of a multi-way system with PSF for AIX.
I hope this helps. If not, let me know and I'll get back with the
performance analyst in Boulder to answer your questions.
S e a r c h - k e y w o r d s:
psf/aix psf/6000 psf aix performance SMP process thread processor
bindprocessor multiple four-way affinity dedicate multithread
thread MP multiprocessor
WWQA: ITEM: RTA000107553 ITEM: RTA000107553
Dated: 01/1999 Category: XPSF6000
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