Messages in this thread | | | Date | Mon, 21 May 2012 16:08:22 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v3 0/2] Move x86_cpu_to_apicid to the __read_mostly section |
| |
* Vlad Zolotarov <vlad@scalemp.com> wrote:
> 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...
Well, a quick tally of percpu variables on a 'make defconfig' kernel would tell us one way or another?
Here there's almost 200 percpu variables active in the 64-bit x86 defconfig, and a quick random sample suggests that most are read-mostly.
I have no fundamental prefer to either approach, but the direction taken should be justified explicitly, with numbers, arguments, etc. - also a short blurb somewhere in the headers that explains when they should be used, so that others can be aware of vSMP's special needs here.
Thanks,
Ingo
| |