Messages in this thread | | | From | Vlad Zolotarov <> | Subject | Re: [PATCH v3 0/2] Move x86_cpu_to_apicid to the __read_mostly section | Date | Mon, 21 May 2012 16:54:25 +0300 |
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On Monday, May 21, 2012 02:32:46 PM Ingo Molnar wrote: > * Shai Fultheim (Shai@ScaleMP.com) <Shai@ScaleMP.com> wrote: > > Ingo, > > > > The reason for this, as you pointed out, is the 'cache line' > > size (4096 bytes). We see significant false sharing is we do > > not move this next to each other. > > Which write-often variable caused the many cache flushes/fills? > cpu_to_apicid is read mostly. > > I.e. it might make more sense to identify the frequenty > *modified* percpu variables, and move them to a separate > section. I *think* most percpu variables are read mostly, so it > would be more maintainable in the long run to figure out the > frequently modified ones, not the frequently not modified ones.
I tend to disagree about the general claim that most per-CPU variables are read-mostly: consider the per-CPU data structures used in lock-less algorithms like softnet_data used in a NAPI. I'm not sure what is a more common - read- only or not-read-only per-cpu data, but surely there are both...
In this specific patch we deal with something that is initialized once in the init time and then used as if it's a constant thus representing a clear "__read_mostly" case.
Having said all that I think that the proposed solution, the one using __read_mostly infrastructure, is just ok both in a long run. I also doubt that we are currently facing a need to define an additional "frequently modified" section.
Pls., comment.
thanks, vlad
> > Thanks, > > Ingo
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