Re: Intel P6 vs P7 system call performance

Linus Torvalds (torvalds@transmeta.com)
Wed, 18 Dec 2002 14:37:35 -0800 (PST)


On Wed, 18 Dec 2002, Jamie Lokier wrote:
>
> That said, you always need the page at 0xfffe0000 mapped anyway, so
> that sysexit can jump to a fixed address (which is fastest).

Yes. This is important. There _needs_ to be some fixed address at least as
far as the kernel is concerned (it might move around between reboots or
something like that, but it needs to be something the kernel knows about
intimately and doesn't need lots of dynamic lookup).

However, there's another issue, namely process startup cost. I personally
want it to be as light as at all possible. I hate doing an "strace" on
user processes and seeing tons and tons of crapola showing up. Just for
fun, do a

strace /bin/sh -c "echo hello"

to see what I'm talking about. And that's actually a _lot_ better these
days than it used to be.

Anyway, I really hate to see "unnecessary crap" in the user mode startup
just because kernel interfaces are bad. That's why I like the AT_SYSINFO
ELF auxilliary table approach - it's something that is already _there_ for
the process to just take advantage of. Having to do a magic mmap for
somehting that everybody needs to do is just bad design.

Linus

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