On Tue, Aug 14, 2007 at 11:26:18AM -0500, Matt Mackall wrote:
> On Tue, Aug 14, 2007 at 04:52:04PM +0800, Fengguang Wu wrote:
> > Hello,
> >
> > Matt Mackall brings us many memory-footprint-optimization
> > opportunities with his pagemap/kpagemap patches. However I wonder if
> > the binary interface can be improved.
> >
> > This seq_file based implementation iterates in the unit of vmas. So
> > super large vmas can be a problem. The next step is to to do address
> > based iteration. That should not be hard.
> >
> > Andrew, please stop me early if it is at all a wrong interface :)
> >
> > Thank you,
> > Fengguang
> > ===
> >
> > Show a process's memory maps page-by-page in /proc/<pid>/pmaps.
> > It helps to analyze application's memory footprint in a comprehensive way.
> >
> > Pages share the same states are grouped into a page range.
> > For each page range, the following fields are exported:
> > - first page index
> > - number of pages in the range
> > - well known page/pte flags
> > - number of mmap users
> >
> > Only page flags not expected to disappear in the near future are exported:
> >
> > Y:young R:referenced A:active U:uptodate P:ptedirty D:dirty W:writeback
> >
> > Here is a sample output:
> >
> > # cat /proc/$$/pmaps
> > 08048000-080c9000 r-xp 08048000 00:00 0
> > 32840 129 Y_A_P__ 1
> > 080c9000-080f6000 rwxp 080c9000 00:00 0 [heap]
> > 32969 45 Y_A_P__ 1
> > f7dba000-f7dc3000 r-xp 00000000 03:00 176633 /lib/libnss_files-2.3.6.so
> > 0 1 Y_AU___ 1
> > 1 1 YR_U___ 1
> > 5 1 YR_U___ 1
> > 8 1 Y_AU___ 1
> > f7dc3000-f7dc5000 rwxp 00008000 03:00 176633 /lib/libnss_files-2.3.6.so
> > 8 2 Y_A_P__ 1
> > f7dc5000-f7dcd000 r-xp 00000000 03:00 176635 /lib/libnss_nis-2.3.6.so
> > 0 1 Y_AU___ 1
> > 1 1 YR_U___ 1
> > 4 1 YR_U___ 1
> > 7 1 Y_AU___ 1
> > f7dcd000-f7dcf000 rwxp 00007000 03:00 176635 /lib/libnss_nis-2.3.6.so
> > 7 2 Y_A_P__ 1
> > f7dcf000-f7de1000 r-xp 00000000 03:00 176630 /lib/libnsl-2.3.6.so
> > 0 1 Y_AU___ 1
> > 1 3 YR_U___ 1
> > 16 1 YR_U___ 1
> > f7de1000-f7de3000 rwxp 00011000 03:00 176630 /lib/libnsl-2.3.6.so
> > 17 2 Y_A_P__ 1
> > [...]
>
> That's a _lot_ of data to generate and parse. I doubt we can watch a
Yes, but won't be too bad because it works in a sparse way.
1) It will only generate output for resident pages, that normally is
much smaller than the mapped size. Take my shell for example, the
(size:rss) ratio is (7:1)!
wfg ~% cat /proc/$$/smaps |grep Size|sum
sum 50552.000
avg 777.723
wfg ~% cat /proc/$$/smaps |grep Rss|sum
sum 7604.000
avg 116.985
2) The page range trick suppresses more output. Look at these lines:
> > 08048000-080c9000 r-xp 08048000 00:00 0
> > 32840 129 Y_A_P__ 1
> > 080c9000-080f6000 rwxp 080c9000 00:00 0 [heap]
> > 32969 45 Y_A_P__ 1
It's interesting to see that the seq_file interface demands some
more programming efforts, and provides this flexibility as well.
3) A 64bit number can be encoded in hex in 8 bytes, no more than its
binary presentation. And often the text string will be much shorter
because of the common small values.
4) String formatting is cheap. Much cheaper than pte/struct page
walkings, and context switches. Not to mention its user friendliness.
> larger app in realtime with this interface. And it doesn't give us
> physical page numbers either.
This could be a problem. Binary interface is discouraging, which is
good and bad. The good thing about it is that we can safely export
the raw PFN/page flag numbers without upsetting upset normal users. In
this way a possible useful kernel debugging interface can be shipped
everywhere.
If kpagemap is a reasonable kernel debug interface, I do not see the
reason why kpagemap and pmaps cannot coexist :)
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