What's a "tahoe"? --- and the new bsd kernel memory allocator

[Chris Torek]

In article <2991 at homxc.UUCP> dwc at 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 at mimsy.umd.edu	Path:	uunet!mimsy!chris