Timers and timing, was: MySQL Performance 6.0rc1

[Julian Elischer]

Poul-Henning Kamp wrote:

>In message <20051028140556.W20147 at fledge.watson.org>, Robert Watson writes:
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>
>>On Fri, 28 Oct 2005, David Xu wrote:
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>>
>>>Poul-Henning Kamp wrote:
>>>      
>>>
>>>>In message <4361FDBE.7000500 at freebsd.org>, David Xu writes:
>>>>
>>>>the correct way to optimize this would be to add a time(2) systemcall 
>>>>which returns the value of the kernel global time_second.
>>>>        
>>>>
>>>Can we make a page in kernel address space which is readable my user 
>>>code? put the variable in the page, I know read an integer is atomic-op, 
>>>needn't lock, so syscall is not needed.
>>>      
>>>
>>This approach has a lot of merit, as we can also potentially export other 
>>information there (such as kernel preferences for system call mechanisms). 
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>>
>
>Yes, there are many advantages to this approach, but we need a solution
>to the API versioning problem before we head that way.
>
>For anyone wanting to look at this, three are a number of nasties
>to remember:
>
>1. How does userland get hold of the page ?  Does it open a magic
>   device ?  Use a magic syscall ?  Or does all processes just get
>   the page by default ?
>
>2. Where in the address space do we put it ?
>  
>

Linux does this and even implements syscalls through it I believe.
We'll probably need to implement it eventually for linux compat.

>3. Layout and alignment issues.  Remember that things change size
>   over time. (Version numbers for each element ?)  And that cross-
>   arch support is desirable (32bit i386 binaries on 64bit amd64 arch)
>
>4. Do guarantee a syscall fallback for all facilities if there is version
>   skew, or do we abort the program ?
>
>5. Do we want a global system page and a per process page while we
>   are at it.  There is plenty of stuff we could put in the per-proc
>   page:  pid, ppid, resource usage, proctitle etc.
>
>
>  
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>>On the other hand, a lower risk change might be to simply add a new CLOCK_ 
>>type for lower resolution, and have a timer synchronize a variable to the 
>>system clock once every 1/10 of a second.  This avoids having to muck with 
>>VM layout, etc.
>>    
>>
>
>Is the CLOCK_* namespace ours to muck about with in the first place ?
>
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