Tom, Yes, you are missing something here. Others have mentioned it, but I will mention it again: Modern processors make extensive use of multi-level cache. When not interrupted, the cache does a great job of improving the performance of the processor by pre-fetching instructions and data post-storing results. The key words here are "not interrupted". Interruptions can occur for normal interrupts in this z/OS system, such as I/O completing, etc., and can occur for other LPARs when the hyper-visor interrupts the processor to dispatch another LPAR. Dedicated LPARs can remove those types of interrupts. But this is exactly the point: the job mix on this LPAR and other processors can and do affect CPU times significantly.
Another fact not well understood is the magnitude of performance degradation when a cache miss occurs. Bob Rogers at the last SHARE mentioned that while cache misses in first and second-level caches were not too bad, cache misses that resulted in actual references to memory could be as much as 600 times slower. His comment: It's almost like doing an I/O to reference main storage compared to getting a hit in first-level cache. You may want to read up on this in the IBM systems journal: http://www.research.ibm.com/journal/rd51-12.html Tom Harper IMS Utilities Development Team Neon Enterprise Software Sugar Land, TX Tom Kelman wrote: Am I missing something here? Miklos is asking about the difference in CPU time between two runs of the same job step. I would think that if the same program was processing the same data in the same way the CPU time should be close to consistent. Maybe not exactly the same but there shouldn't be "several times another". Now the elapsed time could vary widely depending on the contentions that Tom has mentioned above. Tom Kelman Commerce Bank of Kansas City (816) 760-7632 ---------------------------------------------------------------------- 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
