Improving the kernel/i386 timecounter performance (GSoC proposal)

Chuck Swiger cswiger at
Fri Mar 27 13:16:05 PDT 2009

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: 
  [pmap_commpage32_init(), pmap_commpage64_init()] 
  [all :-)] 
  [but this one in particular] 
  [cf "COMM PAGE" comments, vm_commpage_init()] 
  [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... ]


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