The following message is a courtesy copy of an article that has been posted to bit.listserv.ibm-main,alt.folklore.computers as well.
[EMAIL PROTECTED] (Tom Schmidt) writes: > z/VM "waits" with a CPU loop (so it doesn't need to come out of a wait state > when it is waiting) so it would run just as hot when it was idle as > otherwise. > (Unless there is special code to account for machine perspiration?) > Search the archives at http://bama.ua.edu/archives/ibm-main.html arg, if that is true, ... i must have been gone, way too long. virtual machine hypervisor was very careful about "waiting" ... both because of 1) supporting virtual/virtual ... i.e. it might actually be running in a virtual machine ... so it really might be stealing cycles from other applications 2) lot of work had been done in the 60s to make it cost efficient ... somewhat motivated by various customers using platform for commerial timesharing service bureaus. there was obvious work to make the system operate as efficiently as possible ... aka dispatching, scheduling, paging, pathlengths, etc ... as well as making the processing accounting as accurate as possible. however, there was additional features helping make the transition to offering 7x24 availability of online environment. this started in the period when systems were normally leased and processors had "cpu meters" ... and system lease charges were based on value accumulated by the cpu meter. one of the tricks developed ... was making sure that the cpu-meter stopped ... when the system was up and available but otherwise idle. other work was enhancing offshift operation ,,,, when useage might be light or non-existant ... allowing operations w/o onsite operator; aka leave the system up & available for offshift, remote logins ... but otherwise minimize as close to zero as possible, cost of system operation. it wasn't just necessary to put the machine into wait state to stop the cpu meter ... but also quiesce all i/o ... but leave the system available for accepting things like incoming keystrokes. One of the idiosyncrasies of the cpu meter operation was that if it was running and everything stopped ... the meter would continue to run for 400 milliseconds before it actually stopped (i.e. for the cpu meter to actually stop, idle had to be for periods longer than 400 milliseconds). trivia question ... what was the wakeup interval for the mvs srm? ---------------------------------------------------------------------- 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
