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| -rw-r--r-- | lib/README.multi_socket | 115 |
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diff --git a/lib/README.multi_socket b/lib/README.multi_socket new file mode 100644 index 000000000..e18b90497 --- /dev/null +++ b/lib/README.multi_socket @@ -0,0 +1,115 @@ +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. I also + realized that when we use underlying libraries such as c-ares (for DNS + asynch resolving) there might in fact be more than one transfer waiting for + action on the same socket and thus it makes the lookup even tricker and even + less likely to ever get done by applications. Instead I created an internal + "socket to easy handles" hash table that given a socket (file descriptor) + returns a list of easy handles that waits for some 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. |