Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!snorkelwacker!spdcc!ima!esegue!johnl From: johnl@esegue.segue.boston.ma.us (John R. Levine) Newsgroups: comp.arch Subject: Re: Inverted Page Tables Message-ID: <1990Feb22.034517.8676@esegue.segue.boston.ma.us> Date: 22 Feb 90 03:45:17 GMT References: <9708@spool.cs.wisc.edu> <20270@cfctech.cfc.com> <11112@encore.Encore.COM> <753@dgis.dtic.dla.mil> <3606@uceng.UC.EDU> <757@dgis.dtic.dla.mil> <4852@scolex.sco.COM> <29718@brunix.UUCP> <6998@celit.fps.com> <43367@ames.ar Reply-To: johnl@esegue.segue.boston.ma.us (John R. Levine) Organization: Segue Software, Cambridge MA Lines: 26 In article <8106@pt.cs.cmu.edu> lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes: >In article <43367@ames.arc.nasa.gov> lamaster@ames.arc.nasa.gov (Hugh LaMaster) writes: >>1) Several posters have mentioned that there is some unspecified but obvious >>(to them) major problem with using inverted page tables together with >>memory mapped files. ... > >The big problem is with sharing. The ROMP, actually its mmu chip which is named Rosetta, handles this pretty well despite having a reverse map page table with no provision for aliasing. The high four bits of a memory address are looked up in a tiny fast memory to get a segment number. These segment numbers are global to the entire system. The file mapping we put into AIX maps a file to a segment, so if you map a file into several processes there is no aliasing problem; they use the same segment. If you map a file copy-on-write, the aliasing problem does exist, and they handled that at the VRM level via soft page faults. The same problem occurs after a fork, since the pages of the stack and data segments are physically but not logically shared. In that case, they made them copy-on-touch, evidently because by the time they'd handled the soft page fault, the extra effort involved in making a copy of the page wasn't all that great. -- John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 864 9650 johnl@esegue.segue.boston.ma.us, {ima|lotus|spdcc}!esegue!johnl "Now, we are all jelly doughnuts."