Path: utzoo!mnetor!uunet!husc6!mailrus!tut.cis.ohio-state.edu!bgsuvax!gruber From: gruber@bgsuvax.UUCP (John Gruber) Newsgroups: comp.arch Subject: Re: What's a Vax 11/780 MIP really? Message-ID: <1816@bgsuvax.UUCP> Date: 25 Mar 88 17:36:55 GMT References: <413@mn-at1.UUCP> Organization: Bowling Green State University B.G., Oh. Lines: 60 Keywords: Dhrystone IBM 360 370 VAX IMIPS Summary: Differences in the meaning of MIPS; A way to compute a comparison The poster of the referenced article wondered about how many MIPS a VAX 780 ran. Much of the computer industry uses MIPS as a metric for measuring computer processor power; and uses it to compare processors from different families and different manufacturers. If one wasn't interested in knowing how a processor from DEC fared compared to one from, say, IBM, the comparison of a 8600 to a 780 would be sufficient information. Vendors claim that computing MIPS on machines by counting actual instructions executed per second is unfair, because their machine architecture has instructions which do more work than other machine architectures. This sounded like bull to me; but it's not. The number of machine instructions compiled to run the Dhrystone 1.1 benchmark on a VAX is considerably less than the number of IBM 360/370 instructions needed _for the same work_. And it's important to measure work/second rather than something artificial. The Dhrystone 1.1 IBM instructions appeared rather optimal to my trained eye. I don't have the expertise to say how optimal the VAX instructions were. I counted the IBM instructions, since I believe the comparisons to MIPS started there many years ago and that this is what most of the industry is now comparing. If you multiply this number by the ratio of the Dhrystones of two machines, you can develop a IMIPS number how many IBM instructions worth of Dhrystone work a processor does in a second. By running through the benchmark compiled by Dhrystone 1.1, including subroutines, calculating how many times each path is executed, I came up with 593 IBM instructions per Dhrystone 1.1 for the register variant. My best Dhrystone 1.1 measurement for our 780 was 1628. My calculation is that this is .965 IMIPS. The VAX in fact executed a lot fewer instructions to complete the Dhrystones than the IBM computers did. This seemed to be due to the multiple operand instructions, with multiple addressing modes, that tended to replace {load, figure, store result} sets of instructions with just one VAX instruction. I would imagine the difference in measuring a RISC machine in it's native MIPS would demonstrate more dramatically the difficulty of using raw instructions/second measures in comparing different computer architectures. I don't believe in honest-to-goodness-mips. Note that the above IMIPS comparison assumes that Dhrystones are typical of the workload one is interested in running on the machines in question, and also assumes the level of optimization produced by the compilers used in the benchmark. Your workload or compilers may vary. My results were with the 4.3 cc compiler, and a MVS c compiler. Other results: Computer Best Dhrystone 1.1 Perf. IMIPS 785 2018 1.20 IMIPS 4341-2 2407 1.43 IMIPS 8530 7065 4.19 IMIPS 4381-24 5747 3.41 IMIPS (each processor) John Gruber gruber@andy.bgsu.edu tut!bgsuvax!gruber -- John Gruber University Computer Services UUCP:..!cbosgd!osu-eddie!bgsuvax!gruber Bowling Green State University CSNET: gruber%bgsu@csnet-relay Bowling Green, OH 43403-0125