linux-kernel - Re: [Cbe-oss-dev] [RFC, PATCH] CELL Oprofile SPU profiling updated patch

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Date:	Thu, 15 Feb 2007 22:03:59 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Carl Love <cel@...ibm.com>
Cc:	cbe-oss-dev@...abs.org, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org, oprofile-list@...ts.sourceforge.net
Subject: Re: [Cbe-oss-dev] [RFC, PATCH] CELL Oprofile SPU profiling updated	patch

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.

	Arnd <><
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