Messages in this thread | | | Date | Mon, 28 Sep 2009 20:03:17 +0200 | From | Arjan van de Ven <> | Subject | Re: [patch 02/12] Immediate Values - Architecture Independent Code |
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On Mon, 28 Sep 2009 10:46:17 -0700 Andrew Morton <akpm@linux-foundation.org> wrote: > > Kernel gets a lot of cache misses, but that's usually against > userspace, pagecache, net headers/data, etc. I doubt if it gets many > misses against a small number of small, read-mostly data items which > is what this patch addresses. > > And it is a *small* number of things to which this change is > applicable. This is because the write operation for these read-mostly > variables becomes very expensive indeed. This means that we cannot > use "immediate values" for any variable which can conceivable be > modified at high frequency by any workload.
btw just to add to this: caches are unified code/data after L1 in general... it then does not matter much if you encode the "almost constant" in the codestream or slightly farther away, in both cases it takes up cache space. (you can argue "but in the data case it might pull in a whole cacheline just for this".. but that's a case for us to pack such read mostly things properly)
And for L1.. well.. the L2 latency is not THAT much bigger. And L1 is tiny. more icache pressure hurts just as much as having more dcache pressure there.
-- Arjan van de Ven Intel Open Source Technology Centre For development, discussion and tips for power savings, visit http://www.lesswatts.org
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