Re: Intel P6 vs P7 system call performance

["H. Peter Anvin" <hpa@transmeta.com>]

bart@etpmod.phys.tue.nl wrote:
> On 18 Dec, Linus Torvalds wrote:
> 
>>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 
> 
> So why not map the magic page at 0xffffe000 at some other address as
> well? 
> 
> Static binaries can just directly jump/call into the magic page.
> 
> Shared binaries do somekind of mmap("/proc/self/mem") magic to put a
> copy of the page at an address that is convenient for them. Shared
> binaries have to do a lot of mmap-ing anyway, so the overhead should be
> negligible.
> 

That would require /proc to be mounted for all shared binaries to work.
 That is tantamount to killing chroot().

Perhaps it could be done with mremap(), but I would assume that would
entail a special case in the mremap() code.

A special system call would be a bit gross, but it's better than a total
hack.

	-hpa