> -----Original Message----- > From: IBM Mainframe Discussion List [mailto:IBM-MAIN@bama.ua.edu] On > Behalf Of Tom Marchant > Sent: Monday, June 13, 2011 9:52 AM > To: IBM-MAIN@bama.ua.edu > Subject: Re: When is performance really an issue? Was: Running an ISPF > applicction from one pds > > On Sat, 11 Jun 2011 18:56:12 -0400, Farley, Peter wrote: > > >> From: IBM Mainframe Discussion List [mailto:IBM-MAIN@bama.ua.edu] On > >> Behalf Of Lindy Mayfield > >> Sent: Saturday, June 11, 2011 6:28 PM > > > >> Someday I'll take the time to try to understand what "pipeline" means. > >> You guys talk about that a lot. > > You can think about it this way. <Snipped>
Nice, simple explanation. Well done. > >A CPU pipeline is the hardware advance that makes measuring program > >CPU time a gambler's game, since there is no longer any guarantee at > >all that a program will consume the same CPU time from one execution > >to the next, even with totally identical inputs. > > Actually, it is not so much the pipeline that makes the execution times > for an instruction unpredictable, but the variability in the time required > to access memory. Modern processors have small, high speed memory > called cache that the processor can access quickly. If the data is not > there, it has to try a second or third level cache, each of which takes > more time to access. If the data can not be found there, it has to be > fetched from main memory, which is _much_ slower. The processor must > wait for it, and that wait is part of the execution time for the > instruction being processed. Virtual memory translation also takes time, > because it involves even more memory accesses, but Translation Lookaside > Buffers reduce the need to perform page table lookups. But on a nearly idle machine all of those factors should be *nearly* the same if the architecture were predictable. It is no longer predictable or even vaguely consistent is my basic complaint. I'm not whining, it is what it is now and I just deal with it, but it still annoys me when I really want an accurate measurement and all I can get is a statistical average. Peter -- This message and any attachments are intended only for the use of the addressee and may contain information that is privileged and confidential. If the reader of the message is not the intended recipient or an authorized representative of the intended recipient, you are hereby notified that any dissemination of this communication is strictly prohibited. If you have received this communication in error, please notify us immediately by e-mail and delete the message and any attachments from your system. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
