On Mon, 17 Dec 2007 02:33:39 -0800
Andrew Morton <[email protected]> wrote:
> On Fri, 14 Dec 2007 01:33:45 +0100 Andi Kleen <[email protected]> wrote:
>
> > Kyle McMartin <[email protected]> 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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