Re: [PATCH 4/4] maps: /proc/<pid>/pmaps interface - memory maps ingranularity of pages

[Fengguang Wu]

On Sat, Aug 18, 2007 at 12:22:26PM -0500, Matt Mackall wrote:
> > > >     So VSZ:RSS ratio actually goes up with memory pressure.
> > > 
> > > And yes.
> > > 
> > > But that's not what I'm talking about. You're likely to have more
> > > holes in your ranges with memory pressure as things that aren't active
> > > get paged or swapped out and back in. And because we're walking the
> > > LRU more rapidly, we'll flip over a lot of the active bits more often
> > > which will mean more output.
> > > 
> > > >   - page range is a good unit of locality. They are more likely to be
> > > >     reclaimed as a whole. So (RSS:page_ranges) wouldn't degrade as much.
> > > 
> > > There is that. The relative magnitude of the different effects is
> > > unclear. But it is clear that the worst case for pmap is much worse
> > 
> > > than pagemap (two lines per page of RSS?). 
> > It's one line per page. No sane app will make vmas proliferate.
> 
> Sane apps are few and far between.

Very likely, and they will bloat maps/smaps/pmaps alike :(

> > So let's talk about the worst case.
> > 
> > pagemap's data set size is determined by VSZ.
> > 4GB VSZ means 1M PFNs, hence 8MB pagemap data.
> > 
> > pmaps's data set size is bounded by RSS hence physical memory.
> > 4GB RSS means up to 1M page ranges, hence ~20M pmaps data.
> > Not too bad :)
> 
> Hmmm, I've been misreading the output.
> 
> What does it do with nonlinear VMAs?

The implementation gets offset from page_index(page), so will work
the same way in linear/nonlinear VMAs. Depending how one does the
remap_file_ranges() calls, the output lines may be not strictly
ordered by offset, or overlap, or have small page ranges. 

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