Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [Cbe-oss-dev] [RFC, PATCH] CELL Oprofile SPU profiling updated patch | Date | Thu, 15 Feb 2007 22:03:59 +0100 |
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On Thursday 15 February 2007 21:21, Carl Love wrote:
> I have done some quick measurements. The above method limits the loop > to at most 2^16 iterations. Based on running the algorithm in user > space, it takes about 3ms of computation time to do the loop 2^16 times. > > At the vary least, we need to put the resched in say every 10,000 > iterations which would be about every 0.5ms. Should we do a resched > more often?
Yes, just to be on the safe side, I'd suggest to do it every 1000 iterations.
> Additionally we could up the size of the table to 512 which would reduce > the maximum time to about 1.5ms. What do people think about increasing > the table size?
No, that won't help too much. I'd say 256 or 128 entries is the most we should have.
> As for using a logarithmic spacing of the precomputed values, this > approach means that the space between the precomputed values at the high > end would be much larger then 2^14, assuming 256 precomputed values. > That means it could take much longer then 3ms to get the needed LFSR > value for a large N. By evenly spacing the precomputed values, we can > ensure that for all N it will take less then 3ms to get the value. > Personally, I am more comfortable with a hard limit on the compute time > then a variable time that could get much bigger then the 1ms threshold > that Arnd wants for resched. Any thoughts?
When using precomputed values on a logarithmic scale, I'd recommend just rounding to the closest value and accepting the relative inaccuracy, instead of using the precomputed value as the base and then calculating from there.
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