On Sat, 12 Feb 2005, Richard F. Rebel wrote:
>
> That said, many mod_perl users are *VERY* interested in being able to
> detect and observe how "shared" our forked children are. Shared meaning
> private pages shared with children (copy on write). Is it even possible
> to do this in 2.6 kernels? If so, any pointers would be very helpful.
Not in any of the vanilla kernels.
Mauricio has a /proc/<pid>/smaps patch, in which he returns to looking
at every pte slot of every vma of the process as /proc/<pid>/statm did
in 2.4. I suggest you ask him offline for his latest version (the last
I saw did not include support for 2.6.11's pud level; and looped in an
inefficient way, repeatedly locating, mapping and unmapping the page
table for each pte slot - needs refactoring into pgd_range, pud_range,
pmd_range, pte_range levels like 2.4's statm).
It wouldn't be hard to take his framework (before or after improvements),
or the 2.4 proc_pid_statm framework, and modify that to give you the
information you're looking for. But I doubt that the resulting patch
would be accepted into a vanilla kernel - there's no end to the kinds
of numbers that different people might want to see.
I wonder how important swapping is in your case. If it's an
exceptional case that you can ignore, then a "private shared"
entry would be identified by code something like:
if (pte_present(pte)) {
pfn = pte_pfn(pte);
if (pfn_valid(pfn)) {
page = pfn_to_page(pfn);
if (PageAnon(page) && page_mapcount(page) > 1)
private_shared++;
}
}
But if swap use is significant, then it gets rather more complicated:
a present page, even when its mapcount is 1, might still be shared with
other processes, from which which the pte has been unmapped; and where
the pte is not present, there may be a swap entry shared with others.
It can be worked out, but awkward and more than a few lines of code.
Hugh
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