Linus Torvalds wrote:

On Sun, 5 Sep 2004, Arjan van de Ven wrote:

quoted

well... we have a reverse mapping now. What is stopping us from doing
physical defragmentation ?

Nothing but replacement policy, really, and the fact that not everything
is rmappable.

I think we should _normally_ honor replacement policy, the way we do now.  
Only if we are in the situation "we have enough memory, but not enough
high-order-pages" should we go to a separate physical defrag algorithm.

Sure.

So either kswapd should have a totally different mode, or there should be
a separate "kdefragd". It would potentially also be good if it is user-
triggerable, so that you could, for example, have a heavier defragd run
from the daily "cron" runs - something that doesn't seem to make much
sense from a traditional kswapd standpoint.

In other words, I don't think the physical thing should be triggered at 
all by normal memory pressure. A large-order allocation failure would 
trigger it "somewhat", and maybe it might run very slowly in the 
background (wake up every five minutes or so to see if it is worth doing 
anything), and then some user-triggerable way to make it more aggressive.

Does that sound sane to people?

Not to me :P

I think doing it just in time with kswapd and watermarks like we
do for order 0 allocations should be fine.

If you think of kswapd as "do the same freeing work the allocator
will otherwise have to" and "provide a context for doing freeing
work if the allocator can't" (in the !wait case)... then I think my
changes are pretty logical.

I think I confused everybody in the first email - we *do not* try
to heed any order-3 and above watermarks if we're only doing order-2
and below allocations... maybe this was the sticking point?

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