Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!cica!iuvax!ux1.cso.uiuc.edu!csrd.uiuc.edu!s4.csrd.uiuc.edu!turner From: turner@sp1.csrd.uiuc.edu (Steve Turner) Newsgroups: comp.arch Subject: Re: 64 bit sparc ship sets Message-ID: Date: 23 Oct 90 22:37:18 GMT References: <5791@munnari.oz.au> <2774@crdos1.crd.ge.COM> Sender: news@csrd.uiuc.edu (news) Reply-To: turner@csrd.uiuc.edu (Steve Turner) Organization: Center for Supercomputing R & D Lines: 45 In-Reply-To: davidsen@crdos1.crd.ge.COM's message of 18 Oct 90 15:00:03 GMT (Wm E Davidsen Jr) writes: > > (richard oxbrow) writes: > > | [The blurb that i am currently looking at says that it has an > | 8k cache on board (6k instruction and 2k data).] > > Okay, someone enlighten me, why that mix of cache. I'm not sure about the comment about how performance is not hurt by I-cache misses as much as D-cache. I see the point, but jumps that stall the pipeline are still a big performance loss. In any event I think this is highly debatable for the general case. I'll stick my neck out and speculate as to a possible rationale for the sizes. This is a completely intuitive argument, based on several years wasted, er... spent, talking and reading about this stuff. I-cache for typical ISA/load combinations can get very good hit rates (95%+) with a 512 word (2k) cache. This size typically captures most loops, but not whole threads. To get a serious increase in performance beyond this, the I-cache must be increased to capture a significant piece of multiple loops, or most of a thread. In other words, past a certain point, you don't get much "bang-for-the-buck" out of increasing I-cache size. D-cache on the other hand can see almost linear increase in hit rate with increasing size (you can almost feel the breeze from my waving hands at this point) up to quite large sizes. SO: for small (i.e, on-chip) cache systems, you're better off spending most of the real-estate on D- rather than I-cache. I'm suprised no one else has jumped on this already. Perhaps they have, and the post has not made it to our machine yet... -- Steve Turner (on the Si prairie - UIUC CSRD) ARPANET: turner@csrd.uiuc.edu Phone: (217) 244-7293 or (217) 367-0882 I went walking in the wasted city / Started thinking about entropy Smelled the wind from the ruined river / Went home to watch TV -- Warren Zevon