[email protected] (Tom Marchant) writes: > John Eels had a SHARE presentation a couple of years ago where he > described the cost of going to memory. See page 88 of this: > https://share.confex.com/share/119/webprogramschedule/Handout/Session11718/SHARE > 119 Session 11718 Presentation.pdf > > I don't remember what processor this information referred to. Bottom > line is that when the data comes from L1 cache, it is available during > the same machine cycle. If it has to come from main storage, it takes > about 850 machine cycles.
for a decade or so the latency cost for a cache miss to memory counted in processor cycles is similar to the count of 360 processor cycles for a 360 disk i/o ... aka memory has become the new disk. that is motivation for things like hyperthreading (multiple overlapped i-streams) ... simulating multiprocessor ... aka the hardware equivalent of multitasking to allow overlapping execution with things waiting. it is also behind out-of-order execution (skipping past instruction stalled on cache miss). Introduction of out-of-order execution for z196 is claimed to be major factor in the increase in processor throughput between z10 and z196 (something that dates back couple decades in some other platforms). also, processor cycle time has been getting faster than memory latency ... which harkens back to my theme in the 70s and early 80s ... that processor was getting faster, much faster than disks were getting faster. At one point in the early 80s, I was saying that relative system disk speed had declined by a factor of ten times over a period of 15years (processor&memory got 40-50 times, disks got 3-5 times). Disk division executives assigned their performance group to refute my statements ... but after a couple weeks they came back and effectively said that I had slightly understated the problem ... the analysis is respun and turns into a SHARE presentation on optimizing disk configurations for system throughput. -- virtualization experience starting Jan1968, online at home since Mar1970 ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
