Re: Stealing ur megahurts (no, really)

[John Richard Moser]

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Matti Aarnio wrote:
> On Fri, May 19, 2006 at 02:13:02AM -0400, John Richard Moser wrote:
> ...
>> On Linux we have mem= to toy with memory, which I personally HAVE used
>> to evaluate how various distributions and releases of GNOME operate
>> under memory pressure.  This is a lot more convenient than pulling chips
>> and trying to find the right combination.  This option was, apparently,
>> designed for situations where actual system memory capacity is
>> mis-detected (mandrake 7.2 and its insistence that a 256M memory stick
>> is 255M....); but is very useful in this application too.
>>
>> This brings the idea of a cpumhz= parameter to adjust CPU clock rate.
>> Obviously we can't do this directly, as convenient as this would be; but
>> the idea warrants some thought, and some thought I gave it.  What I came
>> up with was simple:  Adjust time slice length and place a delay between
>> time slices so they're evenly spaced.
> ...
>> Questions?  Comments?  Particular ideas on what would happen?
> 
> Modern machines have ability to be "speed controlled" - Perhaps
> they can cut their speed by 1/3 or 1/2, but run slower anyway
> in the name of energy conservation.
> 

Not fine grained enough.  1.8GHz desktop athlon 64 can run at 600MHz or
1.8GHz.  A laptop CPU may run at 2.0GHz, 1.4GHz, 600MHz, and 400MHz.

> 
> Another approach (not thinking on multiprocessor systems now)
> is to somehow gobble up system performance into some "hoarder"
> (highest scheduling priority, eats up 90% of time slices doing
> excellent waste of CPU resources..)

Possible, but could possibly create other issues.

> 
> Combine that with internal timer ticking at 1000 or 1024 Hz, and
> you do get fairly good approximation of a machine running at 1/10
> of its real speed.
> 
> Kernel IO tasks might skew statistics a bit, but that is another story.
> 

Yeah, also a thought.  With my approach you still had interrupts to
account for et al, since on a slow system we should still have <10uS
response time there.

> 
> In multiprocessor systems similar hoarders do work combined with
> CPU Affinity - one hoarder for each processor.
> 
> /Matti Aarnio
> 

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