Messages in this thread |  | | Subject | Re: [PATCH 5/5] slub: Only IPI CPUs that have per cpu obj to flush | | From | Peter Zijlstra <> | | Date | Mon, 26 Sep 2011 11:45:52 +0200 |
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On Mon, 2011-09-26 at 11:35 +0300, Gilad Ben-Yossef wrote: > Yes, the alloc in the flush_all path definitively needs to go. I > wonder if just to resolve that allocating the mask per cpu and not in > kmem_cache itself is not better - after all, all we need is a single > mask per cpu when we wish to do a flush_all and no per cache. The > memory overhead of that is slightly better. This doesn't cover the > cahce bounce issue. > > My thoughts regarding that were that since the flush_all() was a > rather rare operation it is preferable to do some more > work/interference here, if it allows us to avoid having to do more > work in the hotter alloc/dealloc paths, especially since it allows us > to have less IPIs that I figured are more intrusive then cacheline > steals (are they?) > > After all, for each CPU that actually needs to do a flush, we are > making the flush a bit more expensive because of the cache bounce just > before we send the IPI, but that IPI and further operations are an > expensive operations anyway. For CPUs that don't need to do a flush, I > replaced an IPI for a cacheline(s) steal. I figured it was still a > good bargain
Hard to tell really, I've never really worked with these massive machines, biggest I've got is 2 nodes and for that I think your for_each_online_cpu() loop might indeed still be a win when compared to extra accounting on the alloc/free paths.
The problem with a per-cpu cpumask is that you need to disable preemption over the whole for_each_online_cpu() scan and that's not really sane on very large machines as that can easily take a very long time indeed.
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