glibc's malloc() uses arenas [1] in order to efficiently handle memory
allocation of multi-threaded applications. This allows better memory
allocation handling in case of multiple threads that would be using
malloc() concurrently [2].

Due to the python's GIL, this optimization have no effect on
multithreaded python programs. Unfortunately, a downside of creating one
arena per cpu core is the increase of virtual memory which Odoo is based
upon in order to limit the memory usage for threaded workers.

On 32bit systems the default size of an arena is 512K while on 64bit
systems it's 64M [3], hence a threaded worker will quickly reach it's
default memory soft limit upon concurrent requests. We therefore set the
maximum arenas allowed to 2 unless the MALLOC_ARENA_MAX env variable is
set.

This commit also brings the following changes:
- allow to disable the memory hard limit for all servers if the provided
  value is 0 (instead of crashing)
- increase the log level for threaded server in case of limits reached

Note: Setting MALLOC_ARENA_MAX=0 allow to explicitely set the default
      glibs's malloc() behaviour.

[1] https://sourceware.org/glibc/wiki/MallocInternals#Arenas_and_Heaps
[2] https://www.gnu.org/software/libc/manual/html_node/The-GNU-Allocator.html
[3] https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/malloc.c;h=00ce48c;hb=0a8262a#l862



closes odoo/odoo#42323

X-original-commit: 85fe2c6e
Signed-off-by: Christophe Simonis <chs@odoo.com>