Messages in this thread | | | Date | Sat, 28 Feb 2009 16:06:51 -0800 (PST) | Subject | Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache() | From | David Miller <> |
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From: Linus Torvalds <torvalds@linux-foundation.org> Date: Sat, 28 Feb 2009 09:42:18 -0800 (PST)
> On Sat, 28 Feb 2009, Arjan van de Ven wrote: > > > > it invalidates all caches in the hierarchy > > Yeah, now that I look at the intel pdf's, I see that. > > > afaik this is what Intel cpus do; but I also thought this behavior was > > quite architectural as well... > > Ok, I really think we should definitely not use non-temporal stores for > anything smaller than one full page in that case. In fact, I wonder if > even any of the old streaming benchmarks are even true. I thought it would > still stay in the L3, but yes, it literally seems to make the access > totally noncached and WC. > > That's almost unacceptable in the long run. With a 8MB L3 cache - and a > compile sequence, do we really want to go out to memory to write the .S > file, and then have the assembler go out to memory to read it back? For a > compile, that _probably_ is all fine (the compiler in particular will have > enough data structures around that it's not going to fit in the cache > anyway), but I'm seeing leaner compilers and other cases where forcing > things out all the way on the bus is simply the wrong thing.
I think this is an accurate analysis as well, it's really unfortunate the non-temporal stuff on x86 doesn't preserve existing cache lines when present.
I thought that was the whole point. Don't pollute the caches, but if cache lines are already loaded there, use them and don't purge!
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