FreeBSD 9.1 excessive memory allocations

Ian Lepore ian at
Wed Mar 27 14:06:14 UTC 2013

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.

-- Ian

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