RE: About refault distance

["Zhang, Tianfei" <tianfei.zhang@intel.com>]

> 
> On Wed, Jul 16, 2014 at 01:53:55AM +0000, Zhang, Tianfei wrote:
> > Hi Johannes,
> >
> > May I ask you a question about refault distance?
> >
> > Is it supposed the distance of the first and second time to access the
> > a faulted page cache is the same? In reality how about the ratio will be the
> same?
> >
> >             Refault Distance1 = Refault Distance2
> >
> > On the first refault, We supposed that:
> >             Refault Distance = A
> >             NR_INACTIVE_FILE = B
> >             NR_ACTIVE_FILE = C
> >
> > *                  fault page add to inactive list tail
> >                     The Refault Distance  = A
> >                           |
> >  *                   B     |        |            C
> > *              +--------------+   |            +-------------+
> > *   reclaim <- |   inactive   | <-+-- demotion |    active   | <--+
> > *              +--------------+                +-------------+    |
> > *                     |
> |
> > *                     +-------------- promotion ------------------+
> >
> >
> > Why we use A <= C to add faulted page to ACTIVE LIST?
> >
> > Your patch is want to solve "A workload is thrashing when its pages
> > are frequently used but they are evicted from the inactive list every
> > time before another access would have promoted them to the active list." ?
> >
> > so when a First Refault page add to INACTIVE LIST, it is a Distance B before
> eviction.
> > So I am confuse the condition on workingset_refault().
> 
> The reuse distance of a page is B + A.  B + C is the available memory overall.
> When a page refaults, we want to compare its reuse distance to overall
> memory to see if it is eligible for activation (= accessed twice while in memory).
> That check would be A + B <= B + C.  But we can simply drop B on both sides
> and get A <= C.

Thank you very much, it is more clear explanation than comments of code (workingset.c).

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