[collectd] collectd uses a lot of virtual memory

[Christoffer Sawicki]

Hello!

On Fri, Aug 28, 2009 at 10:15, Florian Forster<octo at verplant.org> wrote:
> Hi Christoffer,
>
> On Thu, Aug 27, 2009 at 10:02:52PM +0200, Christoffer Sawicki wrote:
>> VSZ is 67952 which seems a lot to me. Is this normal? Is there anyway
>> I can lower it (perhaps by using more lazy memory allocation)? I care
>> about the amount of allocated memory (and not only used memory) since
>> it is counted against my VPS quota.
>
> yes, this is expected and known behavior: collectd uses a lot of
> threads, which need a (fixed size) stack each. On my Linux system the
> default stack size for threads is 8 MBytes. Since there are at least
> 6 threads in the default configuration, you'd end up with 48+ MBytes of
> virtual memory. 68 MBytes doesn't seem to be far off.

Aha! That explains it. I now remember that I tweaked Apache's stack
size setting some time ago because of the same reason.

> Usually this is not a problem, even vor embedded systems, because Linux
> does overcommitting by default. If this is not an option or, as in your
> case, virtual memory size is limited (why, by the way?),

This is not my domain of expertise, but here's what I've figured:

* Linux instances virtualized by OpenVZ don't have a swap. Since
there's no swap, overcommitting would be dangerous since that could
lead to processes allocating virtual memory that would later be
inaccessible (since there's no real memory backing it). I'm not too
fond of random segfaults.

* OpenVZ Linux instances don't have a swap so that VPS provider
customers need to pay for more RAM. :-)

> you'll have to
> reduce the stack size for new threads. You can do that by setting the
> appropriate ulimit prior to starting collectd, for example in the init-
> script or (under Debian) in /etc/default/collectd.

I use daemontools so I edited my run script, for the record.

I'm thinking that I perhaps should set this setting globally; collectd
is not the only application on my system that uses threads.

> The command would
> look somewhat like this:
>
>  # Set soft stack limit to 512 kBytes
>  $ ulimit -S -s 512
>
> I can't probmise 512 kBytes to be enough for every configuration, but I
> think it should for in most cases. You might even be able to go lower.

I've been running collectd with a stack size of 256 KB for a few hours
now without any problems. The new VSZ (virtual memory size) is 8048
(instead of 67952) -- much friendlier too my system and wallet! :)

Thanks a lot for your answer and a great application.

Best regards,
Christoffer Sawicki