Re: [PATCH 1/2] Make the batch size of the percpu_counter configurable

[Tim Chen <tim.c.chen@linux.intel.com>]

On Tue, 2013-04-30 at 18:27 +0000, Christoph Lameter wrote:
> On Tue, 30 Apr 2013, Tim Chen wrote:
> 
> > > But you will have to dereference the pointer whenever you want the batch
> > > size from the hot path. Looks like it would be better to put the value
> > > there directly. You have a list of percpu counters that can be traversed
> > > to change the batch size.
> > >
> >
> > I have considered that.  But the list is not available unless we have
> > CONFIG_HOTPLUG_CPU compiled in.
> 
> percpu counters are performance sensitive and with the pointer you
> will need to reference another one increasing the cache footprint. You are
> touching an additional cacheline somewhere in memory frequently. Not good.
> 

Seems like there's objection to make batch a pointer and I'll have to
store its value in per cpu pointer.

Will it be acceptable if I make the per cpu counter list under
CONFIG_CPU_HOTPLUG default? I will need the list to go through all
counters to update the batch value sizes.  The alternative will be to
make the configurable batch option only available under
CONFIG_HOTPLUG_CPU.

Which is the preferable approach or there are other clever alternatives?

Tim

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