* Linus Torvalds <torva...@linux-foundation.org> wrote:
> On Wed, Dec 12, 2012 at 2:03 AM, Mel Gorman <mgor...@suse.de> wrote:
> > This is a pull request for "Automatic NUMA Balancing V11". The list
>
> Ok, guys, I've pulled this and pushed out. There were some
> conflicts with both the VM changes and with the scheduler
> tree, but they were pretty small and looked simple, so I fixed
> them up and hope they all work.
Cool, thanks Linus!
> Has anybody tested the impact on single-node systems? If
> distros enable this by default (and it does have 'default y',
> which is a big no-no for new features - I undid that part)
Yes, that was for easy testing, leaving it in was an oversight.
> then there will be tons of people running this without
> actually having multiple sockets. Does it gracefully avoid
> pointless overheads for this case?
Yes. We have:
+ bool numabalancing_default = false;
+
+ if (IS_ENABLED(CONFIG_NUMA_BALANCING_DEFAULT_ENABLED))
+ numabalancing_default = true;
+
+ if (nr_node_ids > 1 && !numabalancing_override) {
+ printk(KERN_INFO "Enabling automatic NUMA balancing. "
+ "Configure with numa_balancing= or sysctl");
+ set_numabalancing_state(numabalancing_default);
+ }
The nr_node_ids check makes sure that on single-node systems we
don't enable the feature.
At that point it will be some extra passive code in the kernel -
last I measured it was around +20K to the kernel image plus a
couple of extra branches in a couple of generic paths - but no
measurable runtime overhead.
Any other negative impact would either come from preparatory or
scalability patches attached to the NUMA balancing feature,
which would be a regression we want to fix.
> Anyway, hopefully we'll have a more real numa balancing for
> 3.9, and this is still considered a reasonable base for that
> work.
We are working on it ;-)
Thanks,
Ingo
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