nginx array utility pool usage

[Maxim Dounin]

Hello!

On Thu, Jan 16, 2014 at 06:22:58PM -0800, Ravi Chunduru wrote:

> Hi Nginx experts,
>   Thanks for the prompt reply to my earlier email on ngx_reset_pool()
> 
> Now, I am looking into ngx_array.c. I found an issue ngx_array_push(). Here
> are the details.
> nginx will check if number of elements is equal to capacity of the array.
> If there is no space in the memory block, it allocates a new memory block
> with twice the size of array and copies over the elements. So far so good.
> Assume that pool utility got entirely new memory block then a->pool is not
> updated with that of  'pool->current'.
> 
> I got an assumption from the code that a->pool is always the memory block
> that has the array elements by seeing the code in ngx_array_push(),
> ngx_array_push_n() or ngx_array_destroy() where checks were always done
> with pool pointer in array.
> 
> Functionalities issues would come up once there is an array overflow. I
> think for every new push of element after first crossing/overflow of the
> capacity, nginx will keep on creating new array. Thus it results in wastage
> of memory.
> 
> Please let me know if its a issue or correct my understanding.

That's expected behaviour.  Arrays are implemented in a way that 
allocates additional memory on overflows, and it's expected to 
happen.  There is a ngx_list_t structure to be used if such 
additional memory allocations are undesired.  Optimization of 
allocations which uses pool internals is just an optimization and 
it's not expected to always succeed.

-- 
Maxim Dounin
http://nginx.org/