Improving the kernel/i386 timecounter performance (GSoC proposal)

[Chuck Swiger]

Hi, Scott & all--

On Mar 27, 2009, at 11:30 AM, Scott Long wrote:
> Robert Watson wrote:
>> On Fri, 27 Mar 2009, Scott Long wrote:
>>> I've been talking about this for years.  All I need is help with  
>>> the VM magic to create the page on fork.  I also want two pages,  
>>> one global for gettimeofday (and any other global data we can  
>>> think of) and one per-process for static data like getpid/getgid.
>> FWIW, there are some variations in schemes across OS's -- one  
>> extreme is the Linux approach, which actually exports a mini shared  
>> library in ELF format on the shared page, providing implementations  
>> of various services (such as entering system calls), time stuff,  
>> etc.  Less extreme are the shared pages offered on Mac OS X, etc.
>
> Yes, but I'd like to start somewhere, and considering that it's been
> impossible in _5_ years to get the 30 minutes of Peter or JeffR or JHB
> time to get the basic VM magic done, I'm keeping my expectations as
> modest as possible.

I'm not entirely sure how close the Mach/xnu and FreeBSD  
implementations of pmap_* stuff are, but the xnu code for commpage  
stuff is here:

http://www.opensource.apple.com/darwinsource/Current/xnu-1228.9.59/osfmk/i386/pmap.c 
  [pmap_commpage32_init(), pmap_commpage64_init()]
http://www.opensource.apple.com/darwinsource/Current/xnu-1228.9.59/osfmk/i386/commpage/ 
  [all :-)]
http://www.opensource.apple.com/darwinsource/Current/xnu-1228.9.59/osfmk/i386/commpage/commpage_gettimeofday.s 
  [but this one in particular]
http://www.opensource.apple.com/darwinsource/Current/xnu-1228.9.59/osfmk/vm/vm_shared_region.c 
  [cf "COMM PAGE" comments, vm_commpage_init()]
http://www.opensource.apple.com/darwinsource/Current/xnu-1228.9.59/bsd/kern/kern_fork.c 
  [fork_create_child(), procdup(), uses of pmap_map_sharedpage()]

[ ADC login might be needed, otherwise I think rwatson has been  
importing xnu periodically for TrustedBSD or other work, and might be  
able to provide similar pointers... ]

Regards,
-- 
-Chuck