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    SubjectRe: RFC: remove __read_mostly
    On Mon, 17 Dec 2007 11:53:36 +0100 Eric Dumazet <> wrote:

    > n Mon, 17 Dec 2007 02:33:39 -0800
    > Andrew Morton <> wrote:
    > > On Fri, 14 Dec 2007 01:33:45 +0100 Andi Kleen <> wrote:
    > >
    > > > Kyle McMartin <> writes:
    > > >
    > > > > I'd bet, in the __read_mostly case at least, that there's no
    > > > > improvement in almost all cases.
    > > >
    > > > I bet you're wrong. Cache line behaviour is critical, much more
    > > > than pipeline behaviour (which unlikely affects). That is because
    > > > if you eat a cache miss it gets really expensive, which e.g.
    > > > a mispredicted jump is relatively cheap in comparison. We're talking
    > > > one or more orders of magnitude.
    > >
    > > So... once we've moved all read-mostly variables into __read_mostly, what
    > > is left behind in bss?
    > >
    > > All the write-often variables. All optimally packed together to nicely
    > > maximise cacheline sharing.
    > This is why it's important to group related variables together, so that they share
    > same cacheline.

    Not feasible. The linker is (I believe) free to place them anywhere it
    likes unless we go and aggregate them in a struct.

    Take (just for one example) inode_lock. How do we prevent that from
    sharing a cacheline with (to pick another example) rtnl_mutex?

    The insidious thing about this is that is is highly dependent upon
    compiler/linker version and upon kernel config. So performance differences
    will appears and disappear with us having very little understanding why.

    I guess we could hunt down the write-very-often variables and put them in
    private cachelines. But there will be a *lot* of them when one considers
    all possible workloads and all possible drivers.

    Now, if we had named it __read_often rather than __read_mostly then we might
    end up with a better result: all those read-mostly, read-rarely variables (and
    there are a lot of those) could be very usefully deployed by packing them
    in between the write-often variables.

    It's crying out for a performance-guided solution.

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