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SubjectRe: [PATCH 1/11] maps3: add proportional set size accounting in smaps
On Mon, Oct 15, 2007 at 04:36:38PM -0700, David Rientjes wrote:
> On Mon, 15 Oct 2007, Matt Mackall wrote:
>
> > Index: l/fs/proc/task_mmu.c
> > ===================================================================
> > --- l.orig/fs/proc/task_mmu.c 2007-10-14 13:35:31.000000000 -0500
> > +++ l/fs/proc/task_mmu.c 2007-10-14 13:36:56.000000000 -0500
> > @@ -122,6 +122,27 @@ struct mem_size_stats
> > unsigned long private_clean;
> > unsigned long private_dirty;
> > unsigned long referenced;
> > +
> > + /*
> > + * Proportional Set Size(PSS): my share of RSS.
> > + *
> > + * PSS of a process is the count of pages it has in memory, where each
> > + * page is divided by the number of processes sharing it. So if a
> > + * process has 1000 pages all to itself, and 1000 shared with one other
> > + * process, its PSS will be 1500. - Matt Mackall, lwn.net
> > + */
> > + u64 pss;
> > + /*
> > + * To keep (accumulated) division errors low, we adopt 64bit pss and
> > + * use some low bits for division errors. So (pss >> PSS_DIV_BITS)
> > + * would be the real byte count.
> > + *
> > + * A shift of 12 before division means(assuming 4K page size):
> > + * - 1M 3-user-pages add up to 8KB errors;
> > + * - supports mapcount up to 2^24, or 16M;
> > + * - supports PSS up to 2^52 bytes, or 4PB.
> > + */
> > +#define PSS_DIV_BITS 12
> > };
> >
>
> I know this gets moved again in the eighth patch of the series, but the
> #define still has no place inside the struct definition.

Agreed.

> The pss is going to need accessor functions, preferably inlined, and the
> comment adjusted stating that all accesses should be through those
> functions and not directly to the mem_size_stats struct.
>
> static inline u64 pss_up(unsigned long pss)
> {
> return pss << PSS_DIV_BITS;
> }
>
> static inline unsigned long pss_down(u64 pss)
> {
> return pss >> PSS_DIV_BITS;
> }

I think that's overkill for something that has exactly one use of each.

--
Mathematics is the supreme nostalgia of our time.
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