Re: Page allocator: Single Zone optimizations

[Christoph Lameter <clameter@sgi.com>]

On Thu, 2 Nov 2006, Mel Gorman wrote:

> Ok... list-based anti-frag identified three types of pages. From the leading
> mail;
> 
> EasyReclaimable - These are userspace pages that are easily reclaimable. This
>         flag is set when it is known that the pages will be trivially
> reclaimed
>         by writing the page out to swap or syncing with backing storage
> 
> KernelReclaimable - These are allocations for some kernel caches that are
>         reclaimable or allocations that are known to be very short-lived.
> 
> KernelNonReclaimable - These are pages that are allocated by the kernel that
>         are not trivially reclaimed. For example, the memory allocated for a
>         loaded module would be in this category. By default, allocations are
>         considered to be of this type
> 
> The EasyReclaimable and KernelReclaimable allocations are marked with __GFP
> flags.
> 
> Now, you want to separate pages according to movable and unmovable. Broadly
> speaking, EasyReclaimable == Movable and
> KernelReclaimable+KernelNonReclaimable == Non-Movable. However, while
> KernelReclaimable are Non-Movable, they can be reclaimed by purging caches.
> So, if we redefined the three terms to be Movable, Reclaimable and
> Non-Movable, you get the separation you are looking for at least within a
> MAX_ORDER_NR_PAGES.

I think talking about reclaim here is not what you want. defragmentation 
is fundamentally about moving memor not reclaim. Reclaim is a way of 
evicting pages from memory to avoid the move. This may be useful if memory 
is filled up because defragging can then do what swapping would have to 
do. However, evicting pages means that they have to be reread. Page 
migration can migrate pages at 1GB/sec which is certainly much higher 
than having to reread the page.

Also I think the reclaim idea breaks down in the following cases:

1. An mlocked page. This is a page that is movable but not reclaimable. 
How does defrag handle that case right now? It should really move the 
page if necessary.

2. There are a number of unreclaimable page types that are easily movable.
F.e. page table pages are movable if you take a write-lock on mmap_sem 
and handle the tree carefully. These pages again are not reclaimable but 
they are movable.

Various caching objects in the slab (cpucache align cache etc) are also 
easily movable. If we put them into a separate slab cache then we could 
make them movable.

Certain Device drivers may be able to shut down intermittendly releasing 
their memory and reallocating it later. This also may be used to move 
memory. Memory allocated by such a device driver is movable.

I would suggest to not categorize pages according to their reclaimability 
but according to their movability. The decision to evict a page (reclaim) 
is something that may be useful to avoid swap but it may be better to keep 
pages in memory.

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