In my Perl script, it runs with high accumulation speed of occupied memory. I have tried making suspect variables clear immediately when they are no longer needed, but the problem can not be fixed. Is there any method to monitor change of memory occupation before and after executing a block?
I have recently had to troubleshoot an out-of-memory situation in one of my programs. While I do not claim to be an expert in this matter by any means, I'm going to share my findings in the hope that it will benefit someone.
1. High, but stable, memory usage
First, you should ensure that you do not just have a case of high, but stable, memory usage. If memory usage is stable, even if your process does not fit in available memory, the discussion below won't be of much help. Here are some notes worth reading in Perl's documentation here and here, in this SO question, in this PerlMonks discussion. There is an interesting analysis here if you're familiar with Perl internals. A lot of deep information is to be found in Tim Bunce's presentation. You should be aware that Perl may not return memory to the system even if you undef stuff. Finally, there's this opinion from a Perl developer that you shouldn't worry too much about memory usage.
2. Steadily growing memory usage
In case memory usage steadily grows, this may eventually cause an out-of-memory situation. My problem turned out to be a case of circular references. According to this answer on StackOverflow, circular references are a common source of memory leaks in Perl. The underlying reason is that Perl uses a reference counting mechanism and cannot release circularly referenced memory until program exit. (Note: I haven't been able to find a more up-to-date version in Perl's documentation of the last claim.)
You can use Scalar::Util::weaken to 'weaken' a circular reference chain (see also http://perlmaven.com/eliminate-circular-reference-memory-leak-using-weaken).
3. Further reading
Tim Bunce's presentation (slides here); also in this blog post
http://www.perlmonks.org/?node_id=472366
Perl memory usage profiling and leak detection?
and of course the link given by #mpapec: http://perlmaven.com/how-much-memory-does-the-perl-application-use
4. Tools
on Unix, you could do system("ps -p $$ -o vsz,rsz,sz,size") Caution: as explained in Tim Bunce's presentation, you'll want to track VSIZE instead of RSS
How to find the amount of physical memory occupied by a hash in Perl?
https://metacpan.org/pod/Devel::Size
and a more recent take by Tim Bunce, which adds the possibility of estimating the total interpreter memory size: https://metacpan.org/pod/Devel::SizeMe
in test scripts, you can use https://metacpan.org/pod/Test::LeakTrace and https://metacpan.org/pod/Test::Memory::Cycle; an example here
https://metacpan.org/pod/Devel::InterpreterSize
Related
I have a C program with perl running as a thread. I would like to restrict the perl interpreter to use memory from a chunk that I pre-allocated (about 2GB). Wonder if it's possible and how to do it.
Thanks.
I'm reasonably certain there is no way to do that in a normal Perl binary, but all perl's memory allocation code is nicely packaged in the malloc.c file in the source code. That file also has lots of comments on how Perl's memory allocation works under the hood. It's shouldn't be too hard to create a locally modified perl that does what you want, I think.
We have a pretty big perl codebase.
Some processes that run for multiple hours (ETL jobs) suddenly started consuming a lot more RAM than usual. Analysis of the changes in the relevant release is a slow and frustrating process. I am hoping to identify the culprit using more automated analysis.
Our live environment is perl 5.14 on Debian squeeze.
I have access to lots of OS X 10.5 machines, though. Dtrace and perl seem to play together nicely on this platform. Seems that using dtrace on linux requires a boot more work. I am hoping that memory allocation patterns will be similar between our live system and a dev OS X system - or at least similar enough to help me find the origin of this new memory use.
This slide deck:
https://dgl.cx/2011/01/dtrace-and-perl
shows how to use dtrace do show number of calls to malloc by perl sub. I am interested in tracking the total amount of memory that perl allocates while executing each sub over the lifetime of a process.
Any ideas on how this can be done?
There's no single way to do this, and doing it on a sub-by-sub basis isn't always the best way to examine memory usage. I'm going to recommend a set of tools that you can use, some work on the program as a whole, others allow you to examine a single section of your code or a single variable.
You might want to consider using Valgrind. There's even a Perl module called Test::Valgrind that will help set up a suppression file for your Perl build, and then check for memory leaks or errors in your script.
There's also Devel::Size which does exactly what you asked for, but on a variable-by-variable basis rather than a sub-by-sub basis.
You can use Devel::Cycle to search for inadvertent circular memory references in complex data structures. While a circular reference doesn't mean that you're wasting memory as you use the object, circular references prevent anything in the chain from being freed until the cycle is broken.
Devel::Leak is a little bit more arcane than the rest, but it basically will allow you to get full information on any SVs that are created and not destroyed between two points in your program's execution. If you check this across a sub call, you'll know any new memory that that subroutine allocated.
You may also want to read the perldebguts section of the Perl manual.
I can't really help more because every codebase is going to wind up being different. Test::Valgrind will work great for some codebases and terribly on others. If you are going to try it, I recommend you use the latest version of Valgrind available and Perl >= 5.10, as Perl 5.8 and Valgrind historically didn't get along too well.
You might want to look at Memory::Usage and Devel::Size
To check the whole process or sub:
use Memory::Usage;
my $mu = Memory::Usage->new();
# Record amount of memory used by current process
$mu->record('starting work');
# Do the thing you want to measure
$object->something_memory_intensive();
# Record amount in use afterwards
$mu->record('after something_memory_intensive()');
# Spit out a report
$mu->dump();
Or to check specific variables:
use Devel::Size qw(size total_size);
my $size = size("A string");
my #foo = (1, 2, 3, 4, 5);
my $other_size = size(\#foo);
my $foo = {
a => [1, 2, 3],
b => {a => [1, 3, 4]}
};
my $total_size = total_size($foo);
The answer to the question is 'yes'. Dtrace can be used to analyze memory usage in a perl process.
This snippet of code:
https://github.com/astletron/perl-dtrace-malloc/blob/master/perl-malloc-total-bytes-by-sub.d
tracks how memory use increases between the call and return of every sub in a program. As an added bonus, dtrace seems to sort the output for you (at least on OS X). Cool.
Thanks to all that chimed in. I answered this one myself as the question is really specific to dtrace/perl.
You could write a simple debug module based on Devel::CallTrace that prints the sub entered as well as the current memory size of the current process. (Using /proc or whatever.)
I am working with Perl for some months now.
As for now, my scripts work but they are far from being perfect.
I would now like to optimize the memory usage, thus I am looking for a way to break down the memory usage per variable/hash.
Is there a way to see how much memory a variable uses?
Devel::Size or Devel::Size::Report can be used to get memory usage for a variable/structure.
You might want to check perl guts illustrated to see what the numbers really mean.
Have a look at Devel::Size on CPAN.
I'm trying to enable the garbage collector of my script to do a better job. There's a ton of memory that it should be able to reclaim, but something is stopping it.
I've used Devel::Cycle a bit and that's allowed me to get closer but I'm not quite there.
How do I find out the current reference count for a Perl hash (the storage for my objects)?
Is there a way to track who is holding a reference to an object? Perhaps a sort of Tie that says, whenever someone points are this object, remember who that someone is.
You are looking for Devel::Refcount.
If you are worried about returning unused memory to the OS, you should know that is not possible in general. The memory footprint of your Perl program will be proportional to the largest allocation during the lifetime of your program.
See How can I make my Perl program take less memory? in the Perl FAQ list as well as Mini-Tutorial: Perl's Memory Management (as pointed out by #Evan Carroll in the comments).
SOLVED see Edit 2
Hello,
I've been writing a Perl program to handle automatic upgrading of local (proprietary) programs (for the company I work for).
Basically, it runs via cron, and unfortunately has a memory leak (or something similar). The problem is that the leak only happens when I'm not looking (aka when run via cron, not via command line).
My code does not contain any circular (or other) references, so the commonly cited tools will not help me (Devel::Cycle, Devel::Peek).
How would I go about figuring out what is using so much memory that the kernel kills it?
Basically, the code SFTPs into a server (using ```sftp...`` `), calls OpenSSL to verify the file, and then SFTPs more if more files are needed, and installs them (untars them).
I have seen delays (~15 sec) before the first SFTP session, but it has never used so much memory as to be killed (in my presence).
If I can't sort this out, I'll need to re-write in a different language, and that will take precious time.
Edit: The following message is printed out by the kernel which led me to believe it was a memory leak:
[100023.123] Out of memory: kill process 9568 (update.pl) score 325406 or a child
[100023.123] Killed Process 9568 (update.pl)
I don't believe it is an issue with cron because of the stalling (for ~15 sec, sometimes) when running it via the command-line. Also, there are no environmental variables used (at least by what I've written, maybe underlying things do?)
Edit 2: I found the issue myself, with help from the below comment by mobrule (in response to this question). It turns out that the script was called from a crontab of a user (non-root) just once a day and that (non-root privs) caused a special infinite loop situation.
Sorry guys, I feel kinda stupid for not finding this before, but thanks.
mobrule, if you submit your comment as an answer, I will accept it as it lead to me finding the problem.
End Edits
Thanks,
Brian
P.S. I may be able to post small snippets of code, but not the whole thing due to company policy.
You could try using Devel::Size to profile some of your objects. e.g. in the main:: scope (the .pl file itself), do something like this:
use Devel::Size qw(total_size);
foreach my $varname (qw(varname1 varname2 ))
{
print "size used for variable $varname: " . total_size($$varname) . "\n";
}
Compare the actual size used to what you think is a reasonable value for each object. Something suspicious might pop out immediately (e.g. a cache that is massively bloated beyond anything that sounds reasonable).
Other things to try:
Eliminate bits of functionality one at a time to see if suddenly things get a lot better; I'd start with the use of any external libraries
Is the bad behaviour localized to just one particular machine, or one particular operating system? Move the program to other systems to see how its behaviour changes.
(In a separate installation) try upgrading to the latest Perl (5.10.1), and also upgrade all your CPAN modules
How do you know that it's a memory leak? I can think of many other reasons why the OS would kill a program.
The first question I would ask is "Does this program always work correctly from the command line?". If the answer is "No" then I'd fix these issues first.
On the other hand if the answer is "Yes", I would investigate all the differences between having the program executed under cron and from the command line to find out why it is misbehaving.
If it is run by cron, that shouldn't it die after iteration? If that is the case, hard for me to see how a memory leak would be a big deal...
Are you sure it is the script itself, and not the child processes that are using the memory? Perhaps it ends up creating a real lot of ssh sessions , instead of doing a bunch of stuff in one session?