FreeBSD 9.1 excessive memory allocations

[Jeremy Chadwick]

On Wed, Mar 27, 2013 at 08:06:10AM -0600, Ian Lepore wrote:
> On Tue, 2013-03-26 at 11:35 -0700, Unga wrote:
> > Hi all
> > 
> > 
> > I have a heavily threaded C application, developed on an Intel Core i5 laptop (2 cores) running FreeBSD 8.1-RELEASE.
> > 
> > When this application compile and run on another Intel Core i7 laptop (4 cores) running FreeBSD 9.1-RELEASE, this application immediately starts grabbing memory by over 100MB per second and soon exit with not enough RAM.
> > 
> > 
> > Both laptops having 4GB RAM.
> > 
> > All malloc and free are mutex locked.
> > 
> > Very rarely this problem happens on the i5 (2 cores) laptop too, but on the i7 laptop, it happens every time.
> > 
> > Appreciate any feedback to identify and fix this issue.
> > 
> > Best regards
> > Unga
> > 
> 
> Too many moving parts, you need to partition the problem.  Is it the
> change in OS (and especially libraries) that causes the problem, or the
> change in the number of cores (more concurrent threads) is exposing some
> sort of application-side race condition?  Given the fact that it does
> occur on 2 cores + freebsd 8.1, even if more rarely, it's almost surely
> an application problem.  
> 
> Perhaps you could use a tool such as valgrind to help track down the
> runaway allocations?
> 
> Another way to expose thread race problems is to force more thread
> context switches.  A blunt tool for doing so is to set hz=5000 or even
> higher in /boot/loader.conf.  I've never done that on a desktop or
> laptop system, only on embedded systems, but it should still work okay.
> If changing the application code is easier, you can get a similar effect
> by creating a thread whose only job is to preempt other threads, by
> using rtprio_thread() to set it to real time priority, then just have it
> sleep for short random intervals (microseconds), all it does is wakes up
> and goes right back to sleep.
> 
> Also, FYI, there's no need to use a mutex in your application to protect
> allocations.  The memory allocator in libc is thread-safe, and in fact
> is particularly designed for efficient multi-threaded allocation.

Thank you Ian -- you've covered nearly all of the points I wanted to
mention in a reply, but opted to not send it (one of those "after I
read it I had second thoughts" situations).

For the OP:

I *strongly* recommend ports/devel/valgrind as mentioned.  It can be a
little bit daunting getting started with it ("okay, wow, what am I doing
with this thing?"), but it's very useful for situations exactly like
this.  There's lots of random tidbits of info on it on the web too.

If the issue turns out to be something in userland (library-wise) or the
kernel, there are FreeBSD.org people who can help with those (I can
think of 4 off the top of my head right now).  They can be CC'd if
things get to that point, but exhaust other avenues first.  It would
also be wise, at that time, if you could make the source to the
application available, that would probably help folks narrow down what's
happening -- otherwise they'll be forced to rely entirely on claims like
"it *should* be behaving like this", which isn't necessarily the same
thing as how the underlying code bits actually behave.

My impression as of this writing, is that the problem is in the
application.  If FreeBSD 9.1 had major problems with memory leaks in
userland threaded apps, I'd expect it would have been discovered by now;
that said, there is always the chance there is a bug somewhere outside
of your application code, it just seems slim.

-- 
| Jeremy Chadwick                                   jdc at koitsu.org |
| UNIX Systems Administrator                http://jdc.koitsu.org/ |
| Mountain View, CA, US                                            |
| Making life hard for others since 1977.             PGP 4BD6C0CB |