Re: RFC: remove __read_mostly

[Eric Dumazet]

On Mon, 17 Dec 2007 02:33:39 -0800
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Fri, 14 Dec 2007 01:33:45 +0100 Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> 
> > Kyle McMartin <kyle@xxxxxxxxxxx> 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.

Random example : vmlist_lock & vmlist

Currently in two separate cache lines (not that important since vmlist is
 so big that one extra cache line access is not measurable)

Other possibilities are :

1) to make sure that really critical hot spots are alone 
(they eventually waste a full cacheline, even if only 4 bytes are in use)

2) Or they are mixed with seldom used data. (One cache line contains one
critical object + other mostly_unused data). This kind of mixing
is really hard to do without a special linker.

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