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

[Fermín Galán Márquez]

Dear Evgeny,

Thanks for your feedback!

Just to check if I have understand correctly how 
MHD_OPTION_THREAD_POOL_SIZE works, let's consider the following example. 
MDH_OPTION_THREAD_POOL_SIZE is set to 4. In a given moment, the MHD 
server is attending at the same time 4 connections at the same time 
(i.e. all the threads in the pool are busy) and a new connection is 
received at the MHD service port. As far as I understand, that 
connection is rejected (i.e. the socket is closed without sending back 
any response to the client).

Is my understanding correct, please?

Thanks!

Best regards,

------

Fermín


El 11/12/2016 a las 20:57, Evgeny Grin escribió:
> 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.