Re: per-process shared information

Andrea Arcangeli
2004-10-15T17:24:19Z

On Fri, Oct 15, 2004 at 12:31:52PM -0400, Albert Cahalan wrote:
> Currently, ps uses /proc/*/stat for that. The /proc/*/statm

maybe you mean /proc/pid/status, status has the RSS and most virtual
info too, I doubt stat has it too (a trivial grep doesn't reveal it at
least).

Anyways my ps definitely reads /proc/*/statm with the 'v' option
(confirmed by strace).

open("/proc/1/stat", O_RDONLY) = 6
read(6, "1 (init) S 0 0 0 0 -1 256 207 16"..., 1023) = 199
close(6) = 0
open("/proc/1/statm", O_RDONLY) = 6
read(6, "147 17 111 111 0 36 0\n", 1023) = 22
close(6) = 0
open("/proc/1/status", O_RDONLY) = 6
read(6, "Name:\tinit\nState:\tS (sleeping)\nS"..., 1023) = 473
close(6) = 0
open("/proc/1/cmdline", O_RDONLY) = 6
read(6, "init [5]\0\0\0", 2047) = 11
close(6) = 0
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(3, 5), ...}) = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo
...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x401cd000
write(1, " PID TTY STAT TIME MAJF"..., 64) = 64
write(1, " 1 ? S 0:02 2"..., 67) = 67
stat64("/proc/2", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
open("/proc/2/stat", O_RDONLY) = 6
read(6, "2 (migration/0) S 1 0 0 0 -1 328"..., 1023) = 135
close(6) = 0

peraphs we use different procps version. I heard there is more than one
version, and I know you're maintaining one but I never followed the
details, you sure know better than me why the above is happening on my
machine. But the point is that there are widely used apps opening statm
(top as you mentioned), and those apps normally don't care about
"shared" (or at least they can't underflow), and those must run on the
big boxes too, and statm was basically unfixable.

> What exactly would be the difference, and when might users see it?

one difference for example is that it cannot take into account for
shared anonymous pages generated by fork(). Or other corner cases would
be posisble, but I believe it's a minor issue... so it's looking good.
Frankly I was thinking shared as a page_mapcount > 1, while all
pagecache is considered "shared", so Hugh's algorithm is a lot closer to
the old shared than what I thought originally. Peraphs solaris also
implements it as rss - anon_rss (I mean, they must be facing similar
issues, and the report states the "shared" info is being reported with
the same semantics on linux 2.4 and solaris and some other unix... so
the theory Hugh's matching other unix even better than 2.4 sounds
reasonable).