Re: [patch 7/9] mm: thrash detection-based file cache sizing

[Johannes Weiner]

On Tue, Jan 14, 2014 at 09:01:09AM +0800, Bob Liu wrote:
> Hi Johannes,
> 
> On 01/11/2014 02:10 AM, Johannes Weiner wrote:
> > The VM maintains cached filesystem pages on two types of lists.  One
> > list holds the pages recently faulted into the cache, the other list
> > holds pages that have been referenced repeatedly on that first list.
> > The idea is to prefer reclaiming young pages over those that have
> > shown to benefit from caching in the past.  We call the recently used
> > list "inactive list" and the frequently used list "active list".
> > 
> > Currently, the VM aims for a 1:1 ratio between the lists, which is the
> > "perfect" trade-off between the ability to *protect* frequently used
> > pages and the ability to *detect* frequently used pages.  This means
> > that working set changes bigger than half of cache memory go
> > undetected and thrash indefinitely, whereas working sets bigger than
> > half of cache memory are unprotected against used-once streams that
> > don't even need caching.
> > 
> 
> Good job! This patch looks good to me and with nice descriptions.
> But it seems that this patch only fix the issue "working set changes
> bigger than half of cache memory go undetected and thrash indefinitely".
> My concern is could it be extended easily to address all other issues
> based on this patch set?
> 
> The other possible way is something like Peter has implemented the CART
> and Clock-Pro which I think may be better because of using advanced
> algorithms and consider the problem as a whole from the beginning.(Sorry
> I haven't get enough time to read the source code, so I'm not 100% sure.)
> http://linux-mm.org/PeterZClockPro2

My patches are moving the VM towards something that is comparable to
how Peter implemented Clock-Pro.  However, the current VM has evolved
over time in small increments based on real life performance
observations.  Rewriting everything in one go would be incredibly
disruptive and I doubt very much we would merge any such proposal in
the first place.  So it's not like I don't see the big picture, it's
just divide and conquer:

Peter's Clock-Pro implementation was basically a double clock with an
intricate system to classify hotness, augmented by eviction
information to work with reuse distances independent of memory size.

What we have right now is a double clock with a very rudimentary
system to classify whether a page is hot: it has been accessed twice
while on the inactive clock.  My patches now add eviction information
to this, and improve the classification so that it can work with reuse
distances up to memory size and is no longer dependent on the inactive
clock size.

This is the smallest imaginable step that is still useful, and even
then we had a lot of discussions about scalability of the data
structures and confusion about how the new data point should be
interpreted.  It also took a long time until somebody read the series
and went, "Ok, this actually makes sense to me."  Now, maybe I suck at
documenting, but maybe this is just complicated stuff.  Either way, we
have to get there collectively, so that the code is maintainable in
the long term.

Once we have these new concepts established, we can further improve
the hotness detector so that it can classify and order pages with
reuse distances beyond memory size.  But this will come with its own
set of problems.  For example, some time ago we stopped regularly
scanning and rotating active pages because of scalability issues, but
we'll most likely need an uptodate estimate of the reuse distances on
the active list in order to classify refaults properly.

> > + * Approximating inactive page access frequency - Observations:
> > + *
> > + * 1. When a page is accessed for the first time, it is added to the
> > + *    head of the inactive list, slides every existing inactive page
> > + *    towards the tail by one slot, and pushes the current tail page
> > + *    out of memory.
> > + *
> > + * 2. When a page is accessed for the second time, it is promoted to
> > + *    the active list, shrinking the inactive list by one slot.  This
> > + *    also slides all inactive pages that were faulted into the cache
> > + *    more recently than the activated page towards the tail of the
> > + *    inactive list.
> > + *
> 
> Nitpick, how about the reference bit?

What do you mean?
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