Re: RFC: remove __read_mostly

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 17 Dec 2007 11:53:36 +0100 Eric Dumazet <dada1@cosmosbay.com> wrote:

> n Mon, 17 Dec 2007 02:33:39 -0800
> Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > On Fri, 14 Dec 2007 01:33:45 +0100 Andi Kleen <andi@firstfloor.org> wrote:
> > 
> > > Kyle McMartin <kyle@mcmartin.ca> 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.