On Sat, 2012-01-28 at 19:50 -0500, Rik van Riel wrote:

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

quoted

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.

I'm aware there's more subtlety, but I think it's a reasonable
generality: your one sentence summary of page_referenced() seems
conspicuously absent; care to provide it ... or would you prefer the VM
internals remain inaccessible to mere mortals? 

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 :)

I didn't give any theories at all about what he should or shouldn't do.
I'm trying to think out loud about whether what he wants and what I
think would help readahead are the same thing (I started of thinking
they were and I talked myself out of it by the end of the previous
email).

quoted

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.

Um, only if you assume you place them at the most recently used head of
the active list ... for obvious reasons, that's not what I was thinking.
I'm still not sure it's more feasible than having separate lists, though
since most recently used tail is  nasty because it's reverse ordering
them and probably not providing sufficient boost and middle insertion
looks just plain wrong.

quoted

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.

quoted

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.

Well, no, that's what I'm trying to argue against.  The chances are that
Streaming RA I/O gets accessed once (the classic movie scenario).  So
the idea is that if you can identify RA as streaming, it should be kept
while unaccessed but discarded after it's been accessed.  To get the LRU
lists to identify this, we want to give a boost to unaccessed unaged RA,
a suppression to accessed once RA and standard heuristics if RA gets
accessed more than once.

James


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