Re: Quick question about hyper-threading (also some NUMA stuff)

[Antonio Vargas]

On Mon, Apr 14, 2003 at 09:37:07AM -0700, Martin J. Bligh wrote:
> >> OK, unless I misunderstand you, I think that happens naturally for that
> >> kind of thing - when we do the COW split, we'll get a node-local page
> >> by default (unless the local node is out of memory).
> >
> > Yes, it happens naturaly, but it's done when we try to write to it.
> > What I meant was, at fork time, it we are forking to a different node,
> > instead of COW-marking, do the COW-mark and the immediately do a sort-of
> > for_each_page(touch_as_if_written(page)), so that nodes would not have to
> > reference the memory from others.
> 
> Ah, you probably don't want to do that ... it's very expensive. Moreover,
> if you exec 2ns later, all the effort will be wasted ... and it's very hard
> to deterministically predict whether you'll exec or not (stupid UNIX 
> semantics). Doing it lazily is probably best, and as to "nodes would not 
> have to reference the memory from others" - you're still doing that, you're
> just batching it on the front end.

True... What about a vma-level COW-ahead just like we have a file-level
read-ahead, then? I mean batching the COW at unCOW-because-of-write time.

btw, COW-ahead sound really silly :)

> > I don't know if it's really usefull, and anyways I could not try to code it
> > unless there is a sort of NUMA simulator for "normal" machines.
> 
> There isn't, but writing one would be very useful (and fairly simple) if
> you have a 2x machine or something that you could use. I've thought about
> writing this several times ... just haven't got round to it.
> 
> M.

Not possible for me since I've got no SMP. But posting a quick note about
your proposed "fake-NUMA-on-SMP.patch" would be good only if to have an
offsite (offbrain also? ;) backup of your ideas :)