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SubjectRe: [PATCH][RFC] simpler __alloc_pages{_limit}
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On August 25, 2001 10:48 am, Mike Galbraith wrote:
> I think the easiest way to handle high order allocations is to do _low_
> volume background reclamation if high order allocations might fail. ie
> put a little effort into keeping such allocations available, but don't
> do massive effort. Cache tends to lose it's value over time, so dumping
> small quantities over time shouldn't hurt.

Ah, time. Please see below for an alternative way of looking at time.

> This would also have the
> benefit of scanning the inactive lists even when there's little activity
> so that list information (page accessed _yesterday_?) won't get stale.

It would probably improve the average performance somewhat. I'm looking at a
more direct approach: if we have a shortage at order(m) then, then for each
(free) member of order(m-1) look in mem_map for its buddy, and if it doesn't
look too active, try to free it, thus moving the allocation unit up one
order. Could you examine this idea please and check for obvious holes?

> I think it's ~fine to reclaim for up to say order 2, but beyond that, it
> doesn't have any up side that I can see.. only down.

Yep.

> btw, I wonder why we don't do memory_pressure [+-]= 1 << order.

We should, at least the "memory_pressure += 1 << order" flavor. Patch?

IMO the concept of memory pressure is bogus. Instead we should calculate the
number of pages to scan directly from the number of pages consumed by
alloc_pages and the probability that deactivated pages will be rescued. The
latter statistic we can measure. We can control it, too, by controlling the
length of the inactive queue. The shorter the inactive queue, the lower the
probability a deactivated page will be rescued.

The idea of clocking recalculate_vm_stats by real time is also bogus. MM
activity does not have a linear relationship to real time under most loads
(exceptions: those loads clocked by a realtime constraint, such as network
bandwidth on a saturated network). We should be clocking the mm using a time
quantum of "one page alloced". A scan of the literature shows considerable
support for this view, and it's also intuitive, don't you think?

What this means in practice is, sure we can have kswapd wake once a second or
once per 100 ms, but the amount of scanning it does needs to be based on the
number of pages actually alloced in that interval. (Intuitively, this
corresponds to the number of free pages needed to replace those alloced.)
>From the mm's perspective, kswapd's constant realtime interval is a variable
delta-t in terms of mm time, meaning that we need to scale all proportional
mm activity by the measured delta. Hmm, clear as mud? This is a standard
idea from control theory.

BTW, what the heck is this supposed to do (page_alloc.c):

144 if (memory_pressure > NR_CPUS)
145 memory_pressure--;
--
Daniel
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