Possible memory leak?

[Maxim Dounin]

Hello!

On Mon, Mar 11, 2019 at 04:37:50PM -0400, wkbrad wrote:

> I think I haven't been clear in what I'm seeing so let's start over.  :)  I
> set up a very simple test on Centos 7 with a default install of Nginx
> 1.12.2.  Below is exactly what I did to produce the result and it's clear to
> me that Nginx is using 2x the ram than it should be using after the first
> reload.  Can anyone explain why the ram usage would double after doing a
> config reload?

As I already tried to explained earlier in this thread, this is a 
result of two things:

1) How nginx allocates memory when doing a configuration reload: 
it creates a new configuration first, and then frees the old one.

2) How system memory allocator works.  Usually it cannot return 
memory to the system if there are any remaining allocations above 
the freed memory regions.  In some cases you can configure system 
allocator to use mmap(), so it will be possible to free such 
allocations, but it may a be a bad idea for other reasons. 

As a result, if large amount of memory is used solely for the 
configuration structures, memory occupied by the nginx master 
process from the system point of view is roughly doubled after a 
configuration reload.

Note that the memory in question is not leaked.  It is properly 
freed by nginx, and it is available for future allocations within 
nginx.  In worker processes, this memory will be used for various 
run-time allocations, such as request buffers and so on.  In the 
master process, this memory will be used on further configuration 
reloads, so the master process will not grow any further.

If the amount of memory used for configuration structures is a 
problem, you may want to re-think your configuration approach.  In 
particular, large virtual hosting providers are known to use nginx 
with small number of server{} blocks serving many different 
domains.  Alternatively, you may want to build nginx with less 
modules compiled in, as each module usually allocates at least 
basic configuration structures in each server{} / location{} even 
if not used.

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