RE: MTXW Status

["Richard Jackson" <richardjackson@xxxxxxxxxxxxxxxxxx>]

Hi Don, long time no talk to :)

Frame of reference issue.  Mutex is the AS/400 name for the thing.  A Mutex
is an implementation of the class semaphore.

There may be a semaphore that is distinct from the mutex in the IPC stuff
(inter-process communication).

They both use hardware instructions named either "test and set" or "compare
and swap".  These are functionally similar atomic instructions that compare
a memory location to a value - for example, is the semaphore S equal to
zero.  If the location equals the value (if S=0), then set S=1 and set a
successful result; else set a failed result.  The AS/400 also supports a
wait value for its compare and swap.  The compare-and-swap functionality is
required for all multi-user, multi-program, multi-CPU, or multi-anything on
a computer.  It is the method used to protect shared memory locations from
simultaneous updates that would corrupt them.

Richard Jackson
mailto:richardjackson@richardjackson.net
www.richardjacksonltd.com
Voice: 1 (303) 808-8058
Fax:   1 (303) 663-4325

-----Original Message-----
From: owner-midrange-l@midrange.com
[mailto:owner-midrange-l@midrange.com]On Behalf Of Schenck, Don
Sent: Thursday, July 20, 2000 8:47 AM
To: 'MIDRANGE-L@midrange.com'
Subject: RE: MTXW Status


Doesn't a semaphore use a mutex?

-----Original Message-----
From: Richard Jackson [mailto:richardjackson@richardjackson.net]
Sent: July 20, 2000 10:14 AM
To: MIDRANGE-L@midrange.com
Subject: RE: MTXW Status


The jobs you see are coded to require access to a particular Mutex before
they can continue.  If they want to run but cannot acquire the Mutex, they
wait.  Mutexes are usually "gates" so that two applications do not
simultaneously update a piece of shared memory.  The holder of the mutex can
update the memory, other programs must wait until they can acquire the mutex
before being allowed to update the shared memory.

Suppose that job 1 and job 2 must acquire mutex X before executing.  Suppose
that job acquires X then job 2 wants X.  Job 2 will wait for X until job 1
releases it.  During this time, job 2 will show mutex-wait.

The reason that they acquire the mutex is known by the jobs and is implicit
in their design but may not be obvious from the outside and might even be
considered confidential by the program designer.  It is possible that that
the party who created the application is holding the mutex longer than they
should - that there is some kind of error.  It is also possible that holding
the mutex for 5 minutes is necessary for the design to work.  It is pretty
difficult to know what is happening from this point of view.  I believe that
this is one reason you sent the note - you wanted to know and you couldn't
figure it out.

At one time, the JDEdwards OneWorld UBE applications used mutexes to
serialize access to certain spec files in the IFS file system.  If that is
the issue, I know a bit more about that - please contact me off line.

Richard Jackson
mailto:richardjackson@richardjackson.net
www.richardjacksonltd.com
Voice: 1 (303) 808-8058
Fax:   1 (303) 663-4325

-----Original Message-----
From: owner-midrange-l@Midrange.com
[mailto:owner-midrange-l@Midrange.com]On Behalf Of Marion, Bob
Sent: Thursday, July 20, 2000 7:43 AM
To: 'Midrange-L@Midrange.com'
Subject: MTXW Status





Any suggestions on the best way to troubleshoot the root cause of jobs going
to a MTXW status on an intermittent basis.   The WRKACTJOB screen shows many
job in a MTXW status for up to 5 minutes on rare occasions, however I have
checked joblogs, performance data. etc.  and can not locate the cause and
during this time the data is not accessible.

The help text for MTXW is as follows

     The initial thread of the job is in a mutex wait.  A mutex is a
     synchronization function that is used to allow multiple jobs or
     processes to serialize their access to shared data.



Bob Marion
EMail:  bob.marion@ps.net


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