| Implementation of the curl_multi_socket API |
| |
| Most of the design decisions and debates about this new API have already |
| been held on the curl-library mailing list a long time ago so I had a basic |
| idea on what approach to use. The main ideas of the new API are simply: |
| |
| 1 - The application can use whatever event system it likes as it gets info |
| from libcurl about what file descriptors libcurl waits for what action |
| on. (The previous API returns fd_sets which is very select()-centric). |
| |
| 2 - When the application discovers action on a single socket, it calls |
| libcurl and informs that there was action on this particular socket and |
| libcurl can then act on that socket/transfer only and not care about |
| any other transfers. (The previous API always had to scan through all |
| the existing transfers.) |
| |
| The idea is that curl_multi_socket() calls a given callback with information |
| about what socket to wait for what action on, and the callback only gets |
| called if the status of that socket has changed. |
| |
| In the API draft from before, we have a timeout argument on a per socket |
| basis and we also allowed curl_multi_socket() to pass in an 'easy handle' |
| instead of socket to allow libcurl to shortcut a lookup and work on the |
| affected easy handle right away. Both these turned out to be bad ideas. |
| |
| The timeout argument was removed from the socket callback since after much |
| thinking I came to the conclusion that we really don't want to handle |
| timeouts on a per socket basis. We need it on a per transfer (easy handle) |
| basis and thus we can't provide it in the callbacks in a nice way. Instead, |
| we have to offer a curl_multi_timeout() that returns the largest amount of |
| time we should wait before we call the "timeout action" of libcurl, to |
| trigger the proper internal timeout action on the affected transfer. To get |
| this to work, I added a struct to each easy handle in which we store an |
| "expire time" (if any). The structs are then "splay sorted" so that we can |
| add and remove times from the linked list and yet somewhat swiftly figure |
| out 1 - how long time there is until the next timer expires and 2 - which |
| timer (handle) should we take care of now. Of course, the upside of all this |
| is that we get a curl_multi_timeout() that should also work with old-style |
| applications that use curl_multi_perform(). |
| |
| The easy handle argument was removed fom the curl_multi_socket() function |
| because having it there would require the application to do a socket to easy |
| handle conversion on its own. I find it very unlikely that applications |
| would want to do that and since libcurl would need such a lookup on its own |
| anyway since we didn't want to force applications to do that translation |
| code (it would be optional), it seemed like an unnecessary option. |
| |
| Instead I created an internal "socket to easy handles" hash table that given |
| a socket (file descriptor) return the easy handle that waits for action on |
| that socket. This hash is made using the already existing hash code |
| (previously only used for the DNS cache). |
| |
| To make libcurl be able to report plain sockets in the socket callback, I |
| had to re-organize the internals of the curl_multi_fdset() etc so that the |
| conversion from sockets to fd_sets for that function is only done in the |
| last step before the data is returned. I also had to extend c-ares to get a |
| function that can return plain sockets, as that library too returned only |
| fd_sets and that is no longer good enough. The changes done to c-ares have |
| been committed and are available in the c-ares CVS repository destined to be |
| included in the upcoming c-ares 1.3.1 release. |
| |
| The 'shiper' tool is the test application I wrote that uses the new |
| curl_multi_socket() in its current state. It seems to be working and it uses |
| the API as it is documented and supposed to work. It is still using |
| select(), because I needed that during development (like until I had the |
| socket hash implemented etc) and because I haven't yet learned how to use |
| libevent or similar. |
| |
| The hiper/shiper tools are very simple and initiates lots of connections and |
| have them running for the test period and then kills them all. |
| |
| Since I wasn't done with the implementation until early January I haven't |
| had time to run very many measurements and checks, but I have done a few |
| runs with up to a few hundred connections (with a single active one). The |
| curl_multi_socket() invoke then takes 3-6 microseconds in average (using the |
| read-only-1-byte-at-a-time hack). If this number does increase a lot when we |
| add connections, it certainly matches my in my opinion very ambitious goal. |
| We are now below the 60 microseconds "per socket action" goal. It is |
| destined to be somewhat higher the more connections we have since the hash |
| table gets more populated and the splay tree will grow etc. |
| |
| Some tests at 7000 and 9000 connections showed that the socket hash lookup |
| is somewhat of a bottle neck. Its current implementation may be a bit too |
| limiting. It simply has a fixed-size array, and on each entry in the array |
| it has a linked list with entries. So the hash only checks which list to |
| scan through. The code I had used so for used a list with merely 7 slots (as |
| that is what the DNS hash uses) but with 7000 connections that would make an |
| average of 1000 nodes in each list to run through. I upped that to 97 slots |
| (I believe a prime is suitable) and noticed a significant speed increase. I |
| need to reconsider the hash implementation or use a rather large default |
| value like this. At 9000 connections I was still below 10us per call. |
| |
| Status Right Now |
| |
| The curl_multi_socket() API is implemented according to how it is |
| documented. |
| |
| http://curl.haxx.se/libcurl/c/curl_multi_socket.html |
| http://curl.haxx.se/libcurl/c/curl_multi_timeout.html |
| http://curl.haxx.se/libcurl/c/curl_multi_setopt.html |
| |
| What is Left for the curl_multi_socket API |
| |
| 1 - More measuring with more extreme number of connections |
| |
| 2 - More testing with actual URLs and complete from start to end transfers. |
| |
| I'm quite sure we don't set expire times all over in the code properly, so |
| there is bound to be some timeout bugs left. |
| |
| What it really takes is for me to commit the code and to make an official |
| release with it so that we get people "out there" to help out testing it. |