Shared Memory Structure?

I am storing things in a shared memory zone allocated with
ngx_shared_memory_add and allocating each slab with
(as these items don’t need to be locked particularly). The problem I’m
having is that I add the shared memory with exactly the amount I need.
However, the final large thing to be allocated fails, but after this a
more smaller things are also allocated without any problem. There is
space for it but it has a problem with a large chunk when space is
Does anyone know why this is or how to fix it? My guess was that nginx
structures the shared memory in a way that means there is enough memory
(which I have made sure there is) but it is not continuous.

Posted at Nginx Forum:


On Tue, May 03, 2016 at 05:45:05AM -0400, ben5192 wrote:

(which I have made sure there is) but it is not continuous.

Shared memory size as specified in a ngx_shared_memory_add() call is the
total size of the memory region to be allocated by nginx. As long
as you use it with slab allocator - various slab allocator
structures are allocated in this memory region, starting with
the ngx_slab_pool_t structure, and followed by multiple
ngx_slab_page_t structures.

If you want to use shared memory for an allocation with a known
size, you have to add some memory to account these internal
structures. Note that it may be not trivial to calculate how may
extra memory you’ll need - see the slab allocator code for
details. Adding at least 8 * ngx_pagesize should be a good

Maxim D.