Messages in this thread | | | Date | Fri, 09 Apr 2004 09:42:02 +1000 | From | Nick Piggin <> | Subject | Re: 2.6.5-rc3-mm4 x86_64 sched domains patch |
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Darren Hart wrote:
>The current default implementations of arch_init_sched_domains >constructs either a flat or two level topolology. The two level >topology is built if CONFIG_NUMA is set. It seems that CONFIG_NUMA is >not the appropriate flag to use for constructing a two level topology >since some architectures which define CONFIG_NUMA would be better served >with a flat topology. x86_64 for example will construct a two level >topology with one CPU per node, causing performance problems because >balancing within nodes is pointless and balancing across nodes doesn't >occur as often. > >
This is correct, although I don't know why there would be performance problems. The rebalance in the degenerate node-local domain should be basically unmeasurable. It would be nice to get rid of it at some time. I have code to prune off degenerate domains, which I will submit soonish.
The NUMA rebalance should occur more often than the old numasched did, but perhaps with some recent Altix-centric changes to the generic setup, this is no longer the case.
The STREAM performance problem is due mainly to the more conservative nature of balancing, which is otherwise a good thing. I think we can fix this in the short term by having x86_64 balance between nodes more often. In the long term, we can merge Ingo's balance on clone stuff, and the interested people can play with that.
>This patch introduces a new CONFIG_SCHED_NUMA flag and uses it to decide >between a flat or two level topology of sched_domains. The patch is >minimally invasive as it primarily modifies Kconfig files and sets the >appropriate default (off for x86_64, on for everything that used to >export CONFIG_NUMA) and should only change the sched_domains topology >constructed on x86_64 systems. I have verified this on a 4 node x86 >NUMAQ, but need someone to test x86_64. > >
I guess I can't see a big problem with this, other than more complexity. In the long run, we should obviously have the arch code set up optimal domains depending on the machine and config.
>This patch is intended as a quick fix for the x86_64 problem, and >doesn't solve the problem of how to build generic sched domain >topologies. We can certainly conceive of various topologies for x86 >systems, so even arch specific topologies may not be sufficient. Would >sub-arch (ie NUMAQ) be the right way to handle different topologies, or >will we be able to autodiscover the appropriate topology? I will be >looking into this more, but thought some might benefit from an immediate >x86_64 fix. I am very interested in hearing your ideas on this. > >
SGI want to do sub arch domains so they can do specific things with their systems. I don't really care what the arch code does with them, but it would be wise to only specialise it when there is a genuine need. I'm glad you'll be looking into it, thanks.
Nick
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