On Tue, Aug 14, 2007 at 09:33:50AM +0800, Fengguang Wu wrote:
> The "proportional set size" (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.
> - lwn.net: "ELC: How much memory are applications really using?"
>
> The PSS proposed by Matt Mackall is a very nice metic for measuring an process's
> memory footprint. So collect and export it via /proc/<pid>/smaps.
>
> Matt Mackall's pagemap/kpagemap and John Berthels's exmap can also do the job,
> providing pretty much details. But for PSS, let's do it in a simple way.
Yes, if people actually want to use this particular metric a lot (and
I obviously personally think it makes a lot of sense), then it should
be done in kernel like this.
> Cc: Matt Mackall <[email protected]>
> Cc: John Berthels <[email protected]>
> Signed-off-by: Fengguang Wu <[email protected]>
> ---
> fs/proc/task_mmu.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> --- linux-2.6.23-rc2-mm2.orig/fs/proc/task_mmu.c
> +++ linux-2.6.23-rc2-mm2/fs/proc/task_mmu.c
> @@ -319,6 +319,7 @@ const struct file_operations proc_maps_o
> struct mem_size_stats
> {
> unsigned long resident;
> + u64 pss; /* proportional set size: my share of rss */
64 bits?
> unsigned long shared_clean;
> unsigned long shared_dirty;
> unsigned long private_clean;
> @@ -341,6 +342,7 @@ static int smaps_pte_range(pmd_t *pmd, u
> pte_t *pte, ptent;
> spinlock_t *ptl;
> struct page *page;
> + int mapcount;
>
> pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> for (; addr != end; pte++, addr += PAGE_SIZE) {
> @@ -357,16 +359,19 @@ static int smaps_pte_range(pmd_t *pmd, u
> /* Accumulate the size in pages that have been accessed. */
> if (pte_young(ptent) || PageReferenced(page))
> mss->referenced += PAGE_SIZE;
> - if (page_mapcount(page) >= 2) {
> + mapcount = page_mapcount(page);
> + if (mapcount >= 2) {
> if (pte_dirty(ptent))
> mss->shared_dirty += PAGE_SIZE;
> else
> mss->shared_clean += PAGE_SIZE;
> + mss->pss += (PAGE_SIZE << 12) / mapcount;
Hmm, what's that shift for? Oh, you're doing fixed-point math.
64-bit divisions are quite expensive on some platforms. The compiler
might be able to do something smarter with common constants like:
if (mapcount == 1)
mss->pss += PAGE_SIZE;
else if (mapcount == 2)
mss->pss += PAGE_SIZE / 2;
else if (mapcount == 3)
mss->pss += PAGE_SIZE / 3;
else if (mapcount == 4)
mss->pss += PAGE_SIZE / 4;
else
mss->pss += PAGE_SIZE / mapcount;
..but I don't know. I suspect we'll at least want to special-case
mapcount == 1 though.
> + sarg.mss.resident >> 10,
> + (unsigned long)(mss->pss >> 22),
And then you're throwing away 22 bits of precision. 10 bits wasn't
enough? Hmmm.. Looks like the worst case is sharing a 4k page 2049
ways, where we'll be off by .999 bytes per 4k page for nearly 50%
error. Your extra 12 bits drops this to .2% error, so I suppose it's
worth it.
But it probably needs a comment.
> - sarg.mss.referenced >> 10);
> + sarg.mss.referenced >> 10);
Unrelated change.
--
Mathematics is the supreme nostalgia of our time.
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