Xref: utzoo comp.arch:6040 comp.unix.wizards:10404 Path: utzoo!utgpu!attcan!uunet!husc6!mailrus!ncar!oddjob!mimsy!chris From: chris@mimsy.UUCP (Chris Torek) Newsgroups: comp.arch,comp.unix.wizards Subject: Re: What's a "tahoe"? --- and the new bsd kernel memory allocator Message-ID: <12957@mimsy.UUCP> Date: 10 Aug 88 19:33:08 GMT References: <6341@bloom-beacon.MIT.EDU> <60859@sun.uucp> <474@m3.mfci.UUCP> <2991@homxc.UUCP> Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742 Lines: 34 In article <2991@homxc.UUCP> dwc@homxc.UUCP (Malaclypse the Elder) writes: >does the tahoe have a more limited kernel address space than the vax? No, in fact, the virtual space is the same size, although it runs from 0xc0000000 to 0xffffffff rather than 0x80000000 to 0xbfffffff. >[in re the 4.3-tahoe kernel malloc, McKusick claimed] that not only was >it not desirable to have internal fragmentation of physical memory This is somewhat debatable. It would be (much) more important if the kernel malloc had a flag telling it to allocate physically contiguous pages, as is required by some of the VAX hardware, e.g. (I keep trying to convince Mike to put this in . . . .) >but [also that] there were systems with limited kernel address space >and that it was undesirable to have fragmentation of virtual address space. >since all the modern processors that i know of (not many) have address >spaces of at least 4Gbytes with at least 1/4 going to kernel, i did >not think this was an issue. While the VAX has a 1 GB kernel space, kernel virtual memory is only mapped by a single level of PTEs, and it takes 1K of physical (PTE) memory to map each 128K of virtual space. A fully populated virtual map would take 8MB of physical memory, which is a bit much. A more reasonable kernel virtual space of (say) 10MB would take a more reasonable 80KB, so that seems feasible. I believe the Tahoe's system space also uses single-level PTEs, while the system space on, e.g., the NS32000 series is two-level. Of course, on machines like the MIPS with software address translation, one can simply use region descriptors. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris