Re: [PATCH] mm: pagevec: cleanup: drop pvec->cold argument in allplaces

From: Andrew Morton
Date: Thu Oct 03 2013 - 17:10:16 EST


On Tue, 1 Oct 2013 22:47:36 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> wrote:

> On Mon, 30 Sep 2013, Andrew Morton wrote:
>
> > > Nobody uses the pvec->cold argument of pagevec and it's also unreasonable for
> > > pages in pagevec released as cold page, so drop the cold argument from pagevec.
> >
> > Is it unreasonable? I'd say it's unreasonable to assume that all pages
> > in all cases are likely to be cache-hot. Example: what if the pages
> > are being truncated and were found to be on the inactive LRU,
> > unreferenced?
> >
> > A useful exercise would be to go through all those pagevec_init() sites
> > and convince ourselves that the decision at each place was the correct
> > one.
> >
>
> Agreed, and the "cold" argument to release_pages() becomes a no-op if this
> patch is merged meaning that anything released through it will
> automatically go to the start of the pcp lists. If the pages aren't hot
> then this is exactly the opposite of what we wanted to do; the fact that
> the pvec length doesn't take into account the size of cpu cache can almost
> guarantee that everything isn't cache hot.

The hot/cold pages code was very marginal when we first merged it and I
suspect it has rotted since.

It would be a useful exercise for someone to disable it then run some
benchmarks with a view to removing it all. But the problem I have with
this approach is perhaps the code *could* become effective if some
careful maintenance work was done on it - we should at least get the
hot/cold decisions optimised before making a decision about the overall
desirability of keeping it.



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