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Started Jan 2000 by Kanoj Sarcar 

Memory balancing is needed for non __GFP_WAIT as well as for non

__GFP_IO allocations.

There are two reasons to be requesting non __GFP_WAIT allocations:

the caller can not sleep (typically intr context), or does not want

to incur cost overheads of page stealing and possible swap io for

whatever reasons.

__GFP_IO allocation requests are made to prevent file system deadlocks.

In the absence of non sleepable allocation requests, it seems detrimental

to be doing balancing. Page reclamation can be kicked off lazily, that

is, only when needed (aka zone free memory is 0), instead of making it

a proactive process.

That being said, the kernel should try to fulfill requests for direct

mapped pages from the direct mapped pool, instead of falling back on

the dma pool, so as to keep the dma pool filled for dma requests (atomic

or not). A similar argument applies to highmem and direct mapped pages.

OTOH, if there is a lot of free dma pages, it is preferable to satisfy

regular memory requests by allocating one from the dma pool, instead

of incurring the overhead of regular zone balancing.

In 2.2, memory balancing/page reclamation would kick off only when the

_total_ number of free pages fell below 1/64 th of total memory. With the

right ratio of dma and regular memory, it is quite possible that balancing

would not be done even when the dma zone was completely empty. 2.2 has

been running production machines of varying memory sizes, and seems to be

doing fine even with the presence of this problem. In 2.3, due to

HIGHMEM, this problem is aggravated.

In 2.3, zone balancing can be done in one of two ways: depending on the

zone size (and possibly of the size of lower class zones), we can decide

at init time how many free pages we should aim for while balancing any

zone. The good part is, while balancing, we do not need to look at sizes

of lower class zones, the bad part is, we might do too frequent balancing

due to ignoring possibly lower usage in the lower class zones. Also,

with a slight change in the allocation routine, it is possible to reduce

the memclass() macro to be a simple equality.

Another possible solution is that we balance only when the free memory

of a zone _and_ all its lower class zones falls below 1/64th of the

total memory in the zone and its lower class zones. This fixes the 2.2

balancing problem, and stays as close to 2.2 behavior as possible. Also,

the balancing algorithm works the same way on the various architectures,

which have different numbers and types of zones. If we wanted to get

fancy, we could assign different weights to free pages in different

zones in the future.

Note that if the size of the regular zone is huge compared to dma zone,

it becomes less significant to consider the free dma pages while

deciding whether to balance the regular zone. The first solution

becomes more attractive then.

The appended patch implements the second solution. It also "fixes" two

problems: first, kswapd is woken up as in 2.2 on low memory conditions

for non-sleepable allocations. Second, the HIGHMEM zone is also balanced,

so as to give a fighting chance for replace_with_highmem() to get a

HIGHMEM page, as well as to ensure that HIGHMEM allocations do not

fall back into regular zone. This also makes sure that HIGHMEM pages

are not leaked (for example, in situations where a HIGHMEM page is in

the swapcache but is not being used by anyone)

kswapd also needs to know about the zones it should balance. kswapd is

primarily needed in a situation where balancing can not be done,

probably because all allocation requests are coming from intr context

and all process contexts are sleeping. For 2.3, kswapd does not really

need to balance the highmem zone, since intr context does not request

highmem pages. kswapd looks at the zone_wake_kswapd field in the zone

structure to decide whether a zone needs balancing.

Page stealing from process memory and shm is done if stealing the page would

alleviate memory pressure on any zone in the page's node that has fallen below

its watermark.

pages_min/pages_low/pages_high/low_on_memory/zone_wake_kswapd: These are

per-zone fields, used to determine when a zone needs to be balanced. When

the number of pages falls below pages_min, the hysteric field low_on_memory

gets set. This stays set till the number of free pages becomes pages_high.

When low_on_memory is set, page allocation requests will try to free some

pages in the zone (providing GFP_WAIT is set in the request). Orthogonal

to this, is the decision to poke kswapd to free some zone pages. That

decision is not hysteresis based, and is done when the number of free

pages is below pages_low; in which case zone_wake_kswapd is also set.

(Good) Ideas that I have heard:

1. Dynamic experience should influence balancing: number of failed requests

for a zone can be tracked and fed into the balancing scheme (jalvo@mbay.net)

2. Implement a replace_with_highmem()-like replace_with_regular() to preserve

dma pages. (lkd@tantalophile.demon.co.uk)