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

[Martin J. Bligh]

>> 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.

> 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.