Chris Friesen wrote:
William Lee Irwin III wrote:
The sorts of like explicit decisions I'd like to be made for these are:
(1) In a mixture of tasks with varying nice numbers, a given nice number
corresponds to some share of CPU bandwidth. Implementations
should not have the freedom to change this arbitrarily according
to some intention.
The first question that comes to my mind is whether nice levels should
be linear or not. I would lean towards nonlinear as it allows a wider
range (although of course at the expense of precision). Maybe something
like "each nice level gives X times the cpu of the previous"? I think a
value of X somewhere between 1.15 and 1.25 might be reasonable.
Nonlinear is a must IMO. I would suggest X = exp(ln(10)/10) ~= 1.2589
That value has the property that a nice=10 task gets 1/10th the cpu of a
nice=0 task, and a nice=20 task gets 1/100 of nice=0. I think that
would be fairly easy to explain to admins and users so that they can
know what to expect from nicing tasks.
What about also having something that looks at latency, and how latency
changes with niceness?
I think this would be a lot harder to pin down, since it's a function of
all the other tasks running and their nice levels. Do you have any of
the RT-derived analysis models in mind?
What about specifying the timeframe over which the cpu bandwidth is
measured? I currently have a system where the application designers
would like it to be totally fair over a period of 1 second. As you can
imagine, mainline doesn't do very well in this case.
It might be easier to specify the maximum deviation from the ideal
bandwidth over a certain period. I.e. something like "over a period of
one second, each task receives within 10% of the expected bandwidth".
- Jim Bruce
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