Re: 2.3.x mem balancing

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Wed, Apr 26, 2000 at 10:24:55AM -0700, Linus Torvalds wrote:
> 
> On Wed, 26 Apr 2000, Rik van Riel wrote:
> > > 
> > > And that subtle issue is that in order for the buddy system to work for
> > > contiguous areas, you cannot have "free" pages _outside_ the buddy system.
> > 
> > This is easy to fix. We can keep a fairly large amount (maybe 4
> > times more than pages_high?) amount of these "free" pages on the
> > queue.
> 
> Note that there are many work-loads that normally have a ton of dirty
> pages. Under those kinds of work-loads it is generally hard to keep a lot
> of "free" pages around, without just wasting a lot of time flushing them
> out all the time.

You have an instant win if the second-chance list is protected by an 
interrupt-safe spinlock.  Do that, and you basically don't ever need 
any free pages at all.  An atomic allocation can go throught the second-
chance list freeing pages until either a buddy page of the required
order becomes available, or we exhaust the list.

With a second-chance list of the same size as our current free page
goals, we'd have exactly the same chance as today of finding a high-
order page.  The advantage would be that our current pessimistic 
free-page management would become truly a lazy reclaim mechanism, 
never freeing a page until it is absolutely necessary.

The cost, of course, is a slightly longer latency while allocating 
memory in interrupts: we've moved some of the kswapd work into the
interrupt itself.  The overall system CPU time, however, should be
reduced if we can avoid unnecessarily freeing pages.

> The other danger with the 'almost free' pages is that it really is very
> load-dependent, and some loads have lots of easily free'd pages.

We have that today.  Whether we are populating the free list or the
last-chance list, we're still having to make that judgement.

--Stephen
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