Messages in this thread | | | Date | Sat, 07 Jan 2006 14:48:22 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH] use local_t for page statistics |
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Andi Kleen wrote: > On Saturday 07 January 2006 04:19, Nick Piggin wrote: > >>Andi Kleen wrote: >> >>>On Saturday 07 January 2006 03:52, Nick Piggin wrote: >>> >>> >>> >>>>No. On many load/store architectures there is no good way to do local_t, >>>>so something like ppc32 or ia64 just uses all atomic operations for >>> >>> >>>well, they're just broken and need to be fixed to not do that. >>> >> >>How? > > > If anything use the 3x duplicated data setup, not atomic operations. >
At a 3x cache footprint cost? (and probably more than 3x for icache, though I haven't checked) And I think hardware trends are against us. (Also, does it have race issues with nested interrupts that Andrew noticed?)
> >>>Also I bet with some tricks a seqlock like setup could be made to work. >>> >> >>I asked you how before. If you can come up with a way then it indeed >>might be a good solution... > > > I'll try to work something up. >
Cool, I'd be interested to see.
> >>The problem I see with seqlock is that it >>is only fast in the read path. That path is not the issue here. > > > The common case - not getting interrupted would be fast. >
The problem is that you can never do the final store without risking a race with an interrupt. Because it is not a read-path.
The closest think I can see to a seqlock would be ll/sc operations, at which point you're back to atomic ops.
> >>>>local_t, and ppc64 uses 3 counters per-cpu thus tripling the cache >>>>footprint. >>> >>> >>>and ppc64 has big caches so this also shouldn't be a problem. >>> >> >>Well it is even less of a problem for them now, by about 1/3. >> >>Performance-wise there is really no benefit for even i386 or x86-64 >>to move to local_t now either so I don't see what the fuss is about. > > > Actually P4 doesn't like CLI/STI. For AMD and P-M it's not that much an issue, > but NetBurst really doesn't like it. >
Yes, it was worth over a second of real time and ~ 7% total kernel time on kbuild on a P4.
(git: a74609fafa2e5cc31d558012abaaa55ec9ad9da4)
AMD and PM I didn't test but the improvement might still be noticable, if much smaller.
-- SUSE Labs, Novell Inc.
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