[PATCH] Mantaining turnstile aligned to 128 bytes in i386 CPUs

[Attilio Rao]

2006/7/25, John Baldwin <jhb at freebsd.org>:
> On Tuesday 25 July 2006 11:14, Attilio Rao wrote:
> > 2006/7/25, Attilio Rao <attilio at freebsd.org>:
> > > Hi,
> > > Intel documentation points out that having a 128-bytes aligned
> > > syncronizing primitive  (which fits in a cache line) will minimize the
> > > traffic for cache bus, so this patch implements an alignment for i386
> > > on turnstiles.
> > >
> > > Any comments, feedbacks?
> >
> > Oh, sorry, I've unforgotten the diff.
> >
> > Attilio
>
> I think a better approach would be to stick turnstiles (and sleepqueues) in a
> UMA zone and specify cache-size alignment to the zone.  However, turnstiles
> aren't really sychronization primitives in that you don't spin on a variable
> inside the structure, and I think it's the spinning and avoiding bouncing
> cache lines around that Intel's documentation is really about.  In that case,
> the things you want aligned are things like mutexes, rwlocks, etc.

Well, I think that this is referred in particular to the latter issue
you mentioned.
Spinning is not really concerned to cache bus issues (more, in
particular, datapath latency).
With this point of view, turnstiles (as sleepqueues) are passed around
CPUs more than a mutex/rwlock (or a cv), I guess, so I was thinking
that it's better optimizing turnstile than the real syncronizing
primitive itself.

Attilio

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