Path: utzoo!attcan!uunet!deimos!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!aglew From: aglew@oberon.csg.uiuc.edu (Andy Glew) Newsgroups: comp.arch Subject: Re: Error in Posting of SPEC numbers on IBM systems Message-ID: Date: 22 Feb 90 16:09:02 GMT References: <36189@mips.mips.COM> <14900004@hpdmd48.HP.COM> Sender: news@ux1.cso.uiuc.edu (News) Organization: University of Illinois, Computer Systems Group Lines: 33 In-Reply-To: sritacco@hpdmd48.HP.COM's message of 21 Feb 90 17:01:55 GMT >From: sritacco@hpdmd48.HP.COM (Steve Ritacco) > >Why is it that the RIOS has a bigger data cache than instruction cache? >This defies conventional wisdom. Data caches are less effective than >instruction caches and are usually made small because their hit ratio >doesn't increase with size as rapidly as instruction cache. If I had to >guess what is going on, I would guess that access to the I-cache is >very wide, to suport super-scaler, so they crammed all they could on >the CPU chip. This seems to be pretty effective. IBM has shown the >super-scaler architecture works, which up to this point I wasn't convinced >of. The benifits are tangible. A 20MgHz CPU with 8K I-cache and 32K >D-cache SPECmarked at 22.something. That is quite impressive. The R3000 >which to date seemed the most efficient CPU/system implementation has >been displaced for the moment. Huh?!? I-caches get better hit rates than D-caches, but you quickly reach a point of diminishing returns - your I-cache hit rate is so good than improving it doesn't make too much difference to your performance. Moreover, if your memory system cycles comparable to your processor, but just has a large latency, then you can suck instructions out of memory, except for branches. Just went to a talk by Mudge of Michigan, who are building a GaAs MIPS 6000, where the speaker had to justify I-cache bigger than D-cache. In this case, they wanted a direct mapped virtual primary D-cache, so the D-cache size was limited by the page size of the architecture, to avoid synonym problems. Since you don't worry about synonyms in the I-cache, the I-cache could be made larger. -- Andy Glew, aglew@uiuc.edu