Messages in this thread | | | Date | Fri, 9 Aug 2002 17:36:15 +0200 | From | Andries Brouwer <> | Subject | Re: Analysis for Linux-2.5 fix/improve get_pid(), comparing various approaches |
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On Fri, Aug 09, 2002 at 07:22:08AM -0400, Hubertus Franke wrote:
> Particulary for large number of tasks, this can lead to frequent exercise of > the repeat resulting in a O(N^2) algorithm. We call this : <algo-0>.
Your math is flawed. The O(N^2) happens only when the name space for pid's has the same order of magnitude as the number N of processes. Now consider N=100000 with 31-bit name space. In a series of 2.10^9 forks you have to do the loop fewer than N times and N^2 / 2.10^9 = 5. You see that on average for each fork there are 5 comparisons. For N=1000000 you rearrange the task list as I described yesterday so that each loop takes time sqrt(N), and altogether N.sqrt(N) comparisons are needed in a series of 2.10^9 forks. That is 0.5 comparisons per fork.
You see that thanks to the large pid space things get really efficient. Ugly constructions are only needed when a large fraction of all possible pids is actually in use, or when you need hard real time guarantees.
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