On Tue, Nov 13, 2012 at 05:40:40PM -0500, Nick Bowler wrote: > On 2012-11-13 14:25 -0800, Paul E. McKenney wrote: > > On Tue, Nov 13, 2012 at 04:47:20PM -0500, Nick Bowler wrote: > > > On 2012-11-13 13:19 -0800, Paul E. McKenney wrote: > > > > On Tue, Nov 13, 2012 at 12:56:54PM -0500, Nick Bowler wrote: > > > > > On 2012-11-13 09:08 -0800, Paul E. McKenney wrote: > > > > > > 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. > > > > > > > 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... Indeed, if I do this, the first step would be to re-introduce the choice between CONFIG_TREE_PREEMPT_RCU and CONFIG_TINY_PREEMPT_RCU so that you could check for horrible explosions. If the past is any guide, there would be at least a few. ;-) Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
