Re: [PATCH] mm: implement WasActive page flag (for improving cleancache)

[Rik van Riel]

On 01/27/2012 04:49 PM, James Bottomley wrote:

> So here, I was just saying your desire to store more data in the page
> table and expand the page flags looks complex.
>
> Perhaps we do have a fundamental misunderstanding:  For readahead, I
> don't really care about the referenced part.  referenced just means
> pointed to by one or more vmas and active means pointed to by two or
> more vmas (unless executable in which case it's one).

That is not at all what "referenced" means everywhere
else in the VM.

If you write theories on what Dan should use, it would
help if you limited yourself to stuff the VM provides
and/or could provide :)

> What I think we care about for readahead is accessed.  This means a page
> that got touched regardless of how many references it has.  An
> unaccessed unaged RA page is a less good candidate for reclaim because
> it should soon be accessed (under the RA heuristics) than an accessed RA
> page.  Obviously if the heuristics misfire, we end up with futile RA
> pages, which we read in expecting to be accessed, but which in fact
> never were (so an unaccessed aged RA page) and need to be evicted.
>
> But for me, perhaps it's enough to put unaccessed RA pages into the
> active list on instantiation and then actually put them in the inactive
> list when they're accessed

That is an absolutely terrible idea for many obvious reasons.

Having readahead pages displace the working set wholesale
is the absolute last thing we want.

> I'm less clear on why you think a WasActive() flag is needed.  I think
> you mean a member of the inactive list that was at some point previously
> active.

> Um, that's complex.  Doesn't your inactive-C list really just identify
> pages that were shared but have sunk in the LRU lists due to lack of
> use?

Nope. Pages that are not mapped can still end up on the active
list, by virtue of getting accessed multiple times in a "short"
period of time (the residence on the inactive list).

We want to cache frequently accessed pages with preference over
streaming IO data that gets accessed infrequently.

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