Possible memory leak?

[Maxim Dounin]

Hello!

On Tue, Mar 12, 2019 at 02:09:06PM -0400, wkbrad wrote:

> First of all, thanks so much for your insights into this and being patient
> with me.  :)  I'm just trying to understand the issue and what can be done
> about it.
> 
> Can you explain to me what you mean by this?
> > you can configure system allocator to use mmap()
> 
> I'm not a C programmer so correct me if I'm wrong, but doesn't the Nginx
> code determine which memory allocator it uses?

Normally C programs use malloc() / free() functions as provided by 
system libc library to allocate memory.  While it is possible for 
an application to provide its own implementation of these 
functions, this is something rarely used in practice.

> If not can you point me to an article that describes how to do that as I
> would like to test it?

For details on how to control system allocator on Linux, please 
refer to the mallopt(3) manpage, notably the 
MALLOC_MMAP_THRESHOLD_ environment variable.  Web version is 
available here:

http://man7.org/linux/man-pages/man3/mallopt.3.html

Please refer to the M_MMAP_THRESHOLD description in the same man 
page for details on what it does and various implications.

Using a values less than NGX_CYCLE_POOL_SIZE (16k by default) 
should help to move all configuration-related allocations into 
mmap(), so these can be freed independently.  Alternatively, 
recompiling nginx with NGX_CYCLE_POOL_SIZE set to a value larger 
than 128k (default mmap() threshold) should have similar 
effect.

Note though that there may be other limiting factors, 
such as MALLOC_MMAP_MAX_, which limits maximum number of mmap() 
allocations to 65536 by default.

You can also play with different allocators by using the 
LD_PRELOAD environment variable, see for example jemalloc's wiki 
here:

https://github.com/jemalloc/jemalloc/wiki/Getting-Started

> Also, you seem to be saying that Nginx IS attempting to free the memory but
> is not able to due to the way the OS is allocating memory or refusing to
> release the memory.  I've tested this in several Linux distros, kernels, and
> Nginx versions and I see the same behavior in all of them.  Do you know of
> an OS or specific distro where Nginx can release the old memory allocations
> correctly?  I would like to test that too.  :)

Any Linux distro can be tuned so freed memory will be returned to 
the system, see above.  And for example on FreeBSD, which uses 
jemalloc as a system allocator, unused memory is properly returned 
to the system out of the box (though can be seen in virtual 
address space occupied by the process, since the allocator uses 
madvise() to make the memory as unused instead of unmapping a 
mapping).

-- 
Maxim Dounin
http://mdounin.ru/