Re: [discuss] Re: 32-bit dma allocations on 64-bit platforms

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Jun 24, 2004 at 10:53:31AM -0700, William Lee Irwin III wrote:
> On Thu, Jun 24, 2004 at 07:39:27PM +0200, Andrea Arcangeli wrote:
> > I looked more into it and you can leave it turned off since it's not
> > going to work.
> > it's all in functions of z->pages_* and those are _global_ for all the
> > zones, and in turn they're absolutely meaningless.
> > the algorithm has nothing in common with lowmem_reverse_ratio, the
> > effect has a tinybit of similarity but the incremntal min thing is so
> > weak and so bad that it will either not help or it'll waste tons of
> > memory. Furthemore you cannot set a sysctl value that works for all
> > machines. The whole thing should be dropped and replaced with the fine
> > production quality lowmem_reserve_ratio in 2.4.26+
> > (the only broken thing of lowmem_reserve_ratio is that it cannot be
> > tuned, not even at boottime, a recompile is needed, but that's fixable
> > to tune it at boot time, and in theory at runtime too, but the point is
> > that no dyanmic tuning is required with it)
> > Please focus on this code of 2.4:
> 
> There is mention of discrimination between pinned and unpinned
> allocations not being possible; I can arrange this for more
> comprehensive coverage if desired. Would you like this to be arranged,
> and if so, how would you like that to interact with the fallback
> heuristics?

how do you handle swapoff and mlock then? anonymous memory is pinned w/o
swap. You've relocate the stuff during the mlock or swapoff to obey to
the pin limits to make this work right, and it sounds quite complicated
and it would hurt mlock performance a lot too (some big app uses mlock
to pagein w/o page faults tons of stuff).

Note that the "pinned" thing in theory makes *perfect* sense, but it
only makes sense on _top_ of lowmem_zone_reserve_ratio, it's not an
alternative.

When the page is pinned you obey to the "lowmem_zone_reserve_ratio" when
it's _not_ pinned then you absolutely ignore the
lowmem_zone_reseve_ratio and you go with the watermarks[curr_zone_idx]
instead of the class_idx.

But in practice I doubt it worth it since I doubt you want to relocate
pagecache and anonymous memory during swapoff/mlock.