Re: PG_zero

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrea Arcangeli wrote:
> On Mon, Nov 01, 2004 at 10:03:56AM -0800, Martin J. Bligh wrote:
> 
>>[..] it was to stop cold
>>allocations from eating into hot pages [..]
> 
> 
> exactly, and I believe that hurts. bouncing on the global lock is going to
> hurt more than preserving an hot page (at least on a 512-way). Plus the
> cold page may very soon become hot too.
> 

Well, the lock isn't global of course. You might be better off
benchmarking on an old Intel 8-way SMP rather than a 512-way Altix :)

But nevertheless I won't say the lock will never hurt.

> Plus you should at least allow an hot allocation to eat into the cold
> pages (which didn't happen IIRC).
> 
> I simply believe using the lru ordering is a more efficient way to
> implement hot/cold behaviour and it will save some minor ram too (with
> big lists the reservation might even confuse the oom conditions, if the
> allocation is hot, but the VM frees in the cold "stopped" list). I know
> the cold list was a lot smaller so this is probably only a theoretical
> issue.
> 

If you don't have cold allocations eating hot pages, nor cold frees
pushing out hot pages then it may be worthwhile.

If that helps a lot, then you couldn't you just have hot allocations
also check the cold list before falling back to the buddy?

I admit I didn't look closely at this - mainly the PG_zero stuff.