NAME
Linux::Smaps - a Perl interface to /proc/PID/smaps
SYNOPSIS
use Linux::Smaps;
my $map=Linux::Smaps->new($pid);
my @vmas=$map->vmas;
my $private_dirty=$map->private_dirty;
...
INSTALLATION
perl Makefile.PL
make
make test
make install
DESCRIPTION
The /proc/PID/smaps files in modern linuxes provides very detailed information about a processes memory consumption. It particularly includes a way to estimate the effect of copy-on-write. This module implements a Perl interface.
The content of the smaps file is a set of blocks like this:
0060a000-0060b000 r--p 0000a000 fd:01 531212 /bin/cat
Size: 4 kB
Rss: 4 kB
Pss: 4 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 4 kB
Referenced: 4 kB
Swap: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Each one describes a virtual memory area of a certain process. All those areas together describe its complete address space. For the meaning of the items refer to your Linux documentation.
The set of information announced by the kernel depends on its version. Early versions (around Linux 2.6.14) lacked for example "Pss", "Referenced", "Swap", "KernelPageSize" and "MMUPageSize". "Linux::Smaps" provides an interface to all of the components. It creates accessor methods dynamically depending on what the kernel reveals. The "Shared_Clean" entry for example mutates to the "Linux::Smaps::VMA->shared_clean" accessor. Method names are built by simply lowercasing them. The actual set of methods is created when the first smaps file is parsed. Subsequent "update" or "Linux::Smaps->new" operations expect exactly the same file format. That means you cannot parse smaps files from different kernel versions by the same perl interpreter.
Constructor, Object Initialization, etc.
Linux::Smaps->new
Linux::Smaps->new($pid)
Linux::Smaps->new(pid=>$pid, procdir=>'/proc')
Linux::Smaps->new(filename=>'/proc/self/smaps')
creates and initializes a "Linux::Smaps" object. On error an exception
is thrown. "new()" may fail if the smaps file is not readable or if the
file format is wrong.
"new()" without parameter is equivalent to "new('self')" or "new(pid=>'self')". With the "procdir" parameter the mount point of the proc filesystem can be set if it differs from the standard "/proc".
The "filename" parameter sets the name of the smaps file directly. This way also files outside the standard "/proc" tree can be analyzed.
Linux::Smaps->new(uninitialized=>1)
returns an uninitialized object. This makes "new()" simply skip the
"update()" call after setting all parameters. Additional parameters like
"pid", "procdir" or "filename" can be passed.
$self->pid($pid) or $self->pid=$pid
$self->procdir($dir) or $self->procdir=$dir
$self->filename($name) or $self->filename=$name
get/set parameters.
If a filename is set "update()" reads that file. Otherwize a file name is constructed from "$self->procdir", "$self->pid" and the name "smaps". The constructed file name is not saved in the "Linux::Smaps" object to allow loops like this:
foreach (@pids) {
$smaps->pid=$_;
$smaps->update;
process $smaps;
}
$self->update
reinitializes the object; rereads the underlying file. Returns the
object or "undef" on error. The actual reason can be obtained via
"lasterror()".
$self->clear_refs
writes to the corresponding /proc/PID/clear_refs file. Thus, the amount
of memory reported as "Referenced" by the process is reset to 0 for all
VMAs.
Returns the object or "undef" on failure.
# how much memory is referenced while updating the Linux::Smaps object?
perl -MLinux::Smaps -le '
my $s=Linux::Smaps->new;
print $s->referenced;
print $s->clear_refs->update->referenced
'
2556
840
# how much memory is used while the shell is inactive?
perl -MLinux::Smaps -le '
my $s=Linux::Smaps->new(shift);
print $s->referenced;
print $s->clear_refs->update->referenced
' $$
1468
0
$self->lasterror
"update()" and "new()" return "undef" on failure. "lasterror()" returns
a more verbose reason. Also $! can be checked.
Information Retrieval
$self->vmas
returns a list of "Linux::Smaps::VMA" objects each describing a vm area,
see below.
$self->size
$self->rss
$self->shared_clean
$self->shared_dirty
$self->private_clean
$self->private_dirty
these methods compute the sums of the corresponding values of all vmas.
"size", "rss", "shared_clean", "shared_dirty", "private_clean" and "private_dirty" methods are unknown until the first call to "Linux::Smaps::update()". They are created on the fly. This is to make the module extendable as new features are added to the smaps file by the kernel. As long as the corresponding smaps file lines match "^(\w+):\s*(\d+) kB$" new accessor methods are created.
At the time of this writing at least one new field ("referenced") is on the way but all my kernels still lack it.
$self->stack
$self->heap
$self->vdso
$self->vsyscall
these are shortcuts to the corresponding "Linux::Smaps::VMA" objects.
$self->all
$self->named
$self->unnamed
In array context these functions return a list of "Linux::Smaps::VMA"
objects representing named or unnamed VMAs or simply all VMAs. Thus, in
array context "all()" is equivalent to "vmas()".
In scalar context these functions create a fake "Linux::Smaps::VMA" object containing the summaries of the "size", "rss", "shared_clean", "shared_dirty", "private_clean" and "private_dirty" fields.
$self->names
returns a list of vma names, i.e. the files that are mapped.
($new, $diff, $old)=$self->diff( $other ) $other is assumed to be also a "Linux::Smaps" instance. 3 arrays are returned. The first one ($new) is a list of vmas that are contained in $self but not in $other. The second one ($diff) contains a list of pairs (2-element arrays) of vmas that differ between $self and $other. The 3rd one ($old) is a list of vmas that are contained in $other but not in $self.
Vmas are identified as corresponding if their "vma_start" fields match. They are considered different if they differ in one of the following fields: "vma_end", "r", "w", "x", "mayshare", "file_off", "dev_major", "dev_minor", "inode", "file_name", "shared_clean", "shared_diry", "private_clean" and "private_dirty".
"Linux::Smaps::VMA" objects
normally these objects represent a single vm area:
$self->vma_start
$self->vma_end
start and end address
$self->r
$self->w
$self->x
$self->mayshare
these correspond to the VM_READ, VM_WRITE, VM_EXEC and VM_MAYSHARE
flags. see Linux kernel for more information.
$self->file_off
$self->dev_major
$self->dev_minor
$self->inode
$self->file_name
describe the file area that is mapped.
$self->size
the same as vma_end - vma_start but in kB.
$self->rss
what part is resident.
$self->shared_clean
$self->shared_dirty
$self->private_clean
$self->private_dirty
"shared" means "page_count(page)>=2" (see Linux kernel), i.e. the page
is shared between several processes. "private" pages belong only to one
process.
"dirty" pages are written to in RAM but not to the corresponding file.
Notes
"size", "rss", "shared_clean", "shared_dirty", "private_clean" and
"private_dirty" methods are unknown until the first call to
"Linux::Smaps::update". They are created on the fly. This is to make the
module extendable as new features are added to the smaps file by the
kernel. As long as the corresponding smaps file lines match
"^(\w+):\s*(\d+) kB$" new accessor methods are created.
At the time of this writing at least one new field ("referenced") is on the way but all my kernels still lack it.
Example: The copy-on-write effect
use strict;
use Linux::Smaps;
my $x="a"x(10241024); # a long string of "a"
if( fork ) {
my $s=Linux::Smaps->new($$);
my $before=$s->all;
$x=~tr/a/b/; # change "a" to "b" in place
#$x="b"x(10241024); # assignment
$s->update;
my $after=$s->all;
foreach my $n (qw{rss size shared_clean shared_dirty
private_clean private_dirty}) {
print "$n: ",$before->$n," => ",$after->$n,": ",
$after->$n-$before->$n,"\n";
}
wait;
} else {
sleep 1;
}
This script may give the following output:
rss: 4160 => 4252: 92
size: 6916 => 7048: 132
shared_clean: 1580 => 1596: 16
shared_dirty: 2412 => 1312: -1100
private_clean: 0 => 0: 0
private_dirty: 168 => 1344: 1176
$x is changed in place. Hence, the overall process size (size and rss) would not grow much. But before the "tr" operation $x was shared by copy-on-write between the 2 processes. Hence, we see a loss of "shared_dirty" (only a little more than our 1024 kB string) and almost the same growth of "private_dirty".
Exchanging the "tr"-operation to an assingment of a MB of "b" yields the following figures:
rss: 4160 => 5276: 1116
size: 6916 => 8076: 1160
shared_clean: 1580 => 1592: 12
shared_dirty: 2432 => 1304: -1128
private_clean: 0 => 0: 0
private_dirty: 148 => 2380: 2232
Now we see the overall process size grows a little more than a MB. "shared_dirty" drops almost a MB and "private_dirty" adds almost 2 MB. That means perl first constructs a 1 MB string of "b". This adds 1 MB to "size", "rss" and "private_dirty" and then copies it to $x. This takes another MB from "shared_dirty" and adds it to "private_dirty".
A special note on copy on write measurements
The proc filesystem reports a page as shared if it belongs multiple processes and as private if it belongs to only one process. But there is an exception. If a page is currently paged out (that means it is not in core) all its attributes including the reference count are paged out as well. So the reference count cannot be read without paging in the page. In this case a page is neither reported as private nor as shared. It is only included in the process size.
Thus, to exaclty measure which pages are shared among N processes at least one of them must be completely in core. This way all pages that can possibly be shared are in core and their reference counts are accessible.
The mlockall(2) syscall may help in this situation. It locks all pages of a process to main memory:
require 'syscall.ph';
require 'sys/mmap.ph';
0==syscall &SYS_mlockall, &MCL_CURRENT | &MCL_FUTURE or
die "ERROR: mlockall failed: $!\n";
This snippet in one of the processes locks it to the main memory. If all processes are created from the same parent it is executed best just before the parent starts to fork off children. The memory lock is not inherited by the children. So all private pages of the children are swappable.
EXPORT
Not an Exporter;
SEE ALSO
Linux Kernel.
AUTHOR
Torsten Foertsch, <torsten.foertsch@gmx.net>
COPYRIGHT AND LICENSE
Copyright (C) 2005-2011 by Torsten Foertsch
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.5 or, at your option, any later version of Perl 5 you may have available.