On 2012-11-13 14:25 -0800, Paul E. McKenney wrote:

On Tue, Nov 13, 2012 at 04:47:20PM -0500, Nick Bowler wrote:

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On 2012-11-13 13:19 -0800, Paul E. McKenney wrote:

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On Tue, Nov 13, 2012 at 12:56:54PM -0500, Nick Bowler wrote:

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On 2012-11-13 09:08 -0800, Paul E. McKenney wrote:

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Suppose that TREE_PREEMPT_RCU was available for !SMP && PREEMPT builds.
Would that work for you?

To be honest I don't really know what the difference is, other than what
the help text says, which is:

  [TINY_PREEMPT_RCU] greatly reduces the memory footprint of RCU.
  
"Greatly reduced memory footprint" sounds pretty useful...

OK, so from your viewpoint, the only possible benefit is smaller
memory?

Well, I have no idea.  If I was given the choice between TREE_PREEMPT_RCU
and TINY_PREEMPT_RCU, absent any information not in the description of
these options, I would choose TINY.  The description suggests that the
memory savings come at the expense of SMP support, which sounds like a
great tradeoff to make for a UP system.

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How much memory does your device have, if I may ask?

It's a (pretty old!) desktop.  I recently had to upgrade it to two
gigabytes due to unbearable thrashing with only one...

If you have two gigabytes (or even one gigabyte), you won't notice the
few kilobytes of difference between TINY_PREEMPT_RCU and TREE_PREEMPT_RCU.

Well then TINY_PREEMPT_RCU doesn't sound all that useful for me!
Perhaps the help text could be improved... such as changing the words
"greatly reduced" to "marginally reduced" as a first step?

Is there no significant cache impact due to the larger implementation?
I don't really have the time or expertise to do measurements in this
regard, but if TREE_PREEMPT_RCU was actually a selectable option I could
at least choose it to see if anything explodes horribly...

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)