Re: cow-ahead N pages for fault clustering

[Martin J. Bligh]

>> > >> 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.
>> > 
>> > That'd be interesting ... and you can test that on a UP box, is not
>> > just NUMA. Depends on the workload quite heavily, I suspect.
>> >  
>> > > btw, COW-ahead sound really silly :)
>> > 
>> > Yeah. So be sure to call it that if it works out ... we need more
>> > things like that ;-) Moooooo.
>> 
>> What about the attached one? I'm compiling it right now to test in UML :)
>> 
>> [ snip fake-NUMA-on-SMP discussion ]
>> 
> 
> OK, too quick for me... this next one applies, compiles and boots on
> 2.5.66 + uml. Now I wonder how can I test if this is useful... ideas?

Well, benchmark it ;-) My favourite trick is to just 
"/usr/bin/time make bzImage" on some fixed kernel version & config,
but aim7 / aim9 is pretty easy to set up too, and might be interesting.

M.