From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of Tom Marchant Sent: Wednesday, March 11, 2009 9:19 AM To: [email protected] Subject: Re: How much does GTF trace impact system throughput?
Bill Fairchild wrote: > >The GTF hook causes a program interrupt Really? I'm surprised. It's not the first time. Tom Marchant It's documented in the PoOps book. Check out MC (Monitor Call). If you are not tracing any events with GTF or any other user of the Monitor Call trace facility, then all the various MC instructions scattered around IBM's code are executed as NOP instructions. There are 16 possible classes of events, such as I/O, SVC, program interrupts, etc. If you have enabled tracing for event class X, then wherever there is a MC instruction that specifies event class X you will get a program interrupt with interrupt code X'0040' when you go through that code. Program Interrupt FLIH checks the interrupt code; if it says the PI was caused by a MC instruction, then you go off into MC interrupt handling code which checks for filtering, etc., as I wrote before. I built a MC trace facility application once before, and it was relatively easy. The tracing routines that are entered are covered by a SETFRR before the MC code calls the tracing routine, so I only rarely crashed my test system.! The only real problem I had was in making it work while GTF was also running. This was back in 1987 before IBM enhanced the MC trace facility so that multiple MC applications could run simultaneously. GTF is but one user of the Monitor Call trace facility. I don't know of any others except the one I built into TMON/MVS, but there is nothing in the MC architecture to prevent a large number of different users of this trace facility from all running simultaneously and tracing the same events. Bill Fairchild Rocket Software ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
