Re: [PATCH 2/3] mm: filemap: only do access activations on reads

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

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On Mon, 2016-04-04 at 14:22 -0700, Andrew Morton wrote:
> On Mon,  4 Apr 2016 13:13:37 -0400 Johannes Weiner <hannes@cmpxchg.or
> g> wrote:
> 
> > 
> > Andres Freund observed that his database workload is struggling
> > with
> > the transaction journal creating pressure on frequently read pages.
> > 
> > Access patterns like transaction journals frequently write the same
> > pages over and over, but in the majority of cases those pages are
> > never read back. There are no caching benefits to be had for those
> > pages, so activating them and having them put pressure on pages
> > that
> > do benefit from caching is a bad choice.
> Read-after-write is a pretty common pattern: temporary files for
> example.  What are the opportunities for regressions here?
> 
> Did you consider providing userspace with a way to hint "this file is
> probably write-then-not-read"?

I suspect the opportunity for regressions is fairly small,
considering that temporary files usually have a very short
life span, and will likely be read-after-written before they
get evicted from the inactive list.

As for hinting, I suspect it may make sense to differentiate
between whole page and partial page writes, where partial
page writes use FGP_ACCESSED, and whole page writes do not,
under the assumption that if we write a partial page, there
may be a higher chance that other parts of the page get
accessed again for other writes (or reads).

I do not know whether that assumption holds :)

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