Re: [PATCH 01 of 16] remove nr_scan_inactive/active

[Rik van Riel <riel@redhat.com>]

Andrew Morton wrote:

>> Scanning fewer pages in the pageout path is probably
>> the way to go.
> 
> I don't see why that would help.  The bottom-line steady-state case is that
> we need to reclaim N pages per second, and we need to scan N*M vmas per
> second to do so.  How we chunk that up won't affect the aggregate amount of
> work which needs to be done.
> 
> Or maybe you're referring to the ongoing LRU balancing thing.  Or to something
> else.

Yes, I am indeed talking about LRU balancing.

We pretty much *know* that an anonymous page on the
active list is accessed, so why bother scanning them
all?

We could just deactivate the oldest ones and clear
their referenced bits.

Once they reach the end of the inactive list, we
check for the referenced bit again.  If the page
was accessed, we move it back to the active list.

The only problem with this is that anonymous
pages could be easily pushed out of memory by
the page cache, because the page cache has
totally different locality of reference.

The page cache also benefits from the use-once
scheme we have in place today.

Because of these three reasons, I want to split
the page cache LRU lists from the anonymous
memory LRU lists.

Does this make sense to you?

>> No matter how efficient we make the scanning of one
>> individual page, we simply cannot scan through 1TB
>> worth of anonymous pages (which are all referenced
>> because they've been there for a week) in order to
>> deactivate something.
> 
> Sure.  And we could avoid that sudden transition by balancing the LRU prior
> to hitting the great pages_high wall.

Yes, we will need to do some preactive balancing.

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