Re: [libmicrohttpd] Doubts regarding how MHD_OPTION_THREAD_POOL_SIZE and MHD_OPTION_CONNECTION_LIMIT interact together

[Evgeny Grin]

Hi Fermin,

First of all, I'd like to clarify: MHD_OPTION_THREAD_POOL_SIZE is
supposed to be set to value equal or close to number of available CPU
cores. If you set MHD_OPTION_THREAD_POOL_SIZE to value higher than
number of CPU cores then you'll most probably experience performance
decrease: number of actively executed threads at the same time will be
limited by number of CPU cores and system will waste resources for
switching threads.

For other answers see below.

On 07.12.2016 14:15, Fermín Galán Márquez wrote:
> Assuming that MHD_OPTION_THREAD_POOL_SIZE is greater than zero (i.e.
> thread pool is actually used) the following posibilites are possible:
> 
>   * MHD_OPTION_THREAD_POOL_SIZE => MHD_OPTION_CONNECTION_LIMIT. In this
>     case, I understand that MHD_OPTION_CONNECTION_LIMIT limits the
>     number of concurrent connections, no matter the size of the pool.
Correct.

>   * MHD_OPTION_THREAD_POOL_SIZE < MHD_OPTION_CONNECTION_LIMIT. Not sure
>     what happen in this case if all the threads in the pool are busy and
>     a new the process receives a new incoming connection, among the
>     following:
>      1. The connection is rejected (as if it has hit the
>         MHD_OPTION_CONNECTION_LIMIT)
>      2. The connection is somehow enqueued internally until a free
>         thread in the pool can deal with it. In that case I understand
>         that MHD_OPTION_CONNECTION_LIMIT is the size of such internal queue.
>      3. A new "extra to pool" thread is created to deal with the
>         incoming connection (to be destroyed once the requests has been
>         processed).
> 
> Which option corresponds to the actual behaviour, please?

The same as in first case. Number of connections and size of thread pool
are independent.

When MHD instance reach specified connection limits, next connection
could be ether rejected explicitly or not accepter before some
connections are closed (actually both is possible and cannot be
configured). In any case - MHD will not process more connection at the
same time then configured by MHD_OPTION_CONNECTION_LIMIT.

-- 
Best Wishes,
Evgeny Grin