Re: Thread Parallelism for Sockets (Fix for infinite loops/deadlocks in NSE)

["Patrick Donnelly" <batrick.donnelly () gmail com>]

On Thu, Jul 24, 2008 at 3:58 AM, Patrick Donnelly
<batrick.donnelly () gmail com> wrote:
> Some technical details of the implementation:
>
> 1) The open sockets are paired with a per thread unique userdata
> (henceforth called 'proxies') in a weak keyed table. When all sockets
> have closed or been collected, the userdata is collected and a slot is
> freed in a fully weak table with thread proxy pairs (which contains
> the number of threads with open sockets). When the proxy is collected
> (and thus a slot freed), a thread waiting to open a socket is moved to
> running and given a lock.

Here is the commented documentation on this, which may clear up any confusion:

<code>

/* Some constants used for enforcing a limit on the number of open sockets
 * in use by threads. The maximum value between MAX_PARALLELISM and
 * o.maxparallelism is the max # of threads that can have connected sockets
 * (open). THREAD_PROXY, SOCKET_PROXY, and CONNECT_WAITING are tables in the
 * nsock C functions' environment, at LUA_ENVIRONINDEX, that hold sockets and
 * threads used to enforce this. THREAD_PROXY has <Thread, Userdata> pairs
 * that associate a thread to a proxy userdata. This table has weak keys and
 * values so threads and the proxy itself can be collected. SOCKET_PROXY
 * has <Socket, Userdata> pairs that associate a socket to a proxy userdata.
 * SOCKET_PROXY has weak keys (to allow the collection of sockets) and strong
 * values, so the proxies are not collected when an associated socket is open.
 *
 * All the sockets used by a thread have the same Proxy Userdata. When all
 * sockets in use by a thread are closed or collected, the entry in the
 * THREAD_PROXY table is cleared, freeing up a slot for another thread
 * to make connections. When a slot is freed, proxy_gc is called, via the
 * userdata's __gc metamethod, which will add a thread in WAITING to running.
 */
#define MAX_PARALLELISM   10
#define THREAD_PROXY       1 /* <Thread, Userdata> */
#define SOCKET_PROXY       2 /* <Socket, Userdata> */
#define CONNECT_WAITING    3 /* Threads waiting to lock */
#define PROXY_META         4 /* Proxy userdata's metatable */

</code>

Cheers,

-- 
-Patrick Donnelly

"One of the lessons of history is that nothing is often a good thing
to do and always a clever thing to say."

-Will Durant

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