74 lines
3.8 KiB
Plaintext
74 lines
3.8 KiB
Plaintext
Implementation of the curl_multi_socket API
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The main ideas of the new API are simply:
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1 - The application can use whatever event system it likes as it gets info
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from libcurl about what file descriptors libcurl waits for what action
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on. (The previous API returns fd_sets which is very select()-centric).
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2 - When the application discovers action on a single socket, it calls
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libcurl and informs that there was action on this particular socket and
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libcurl can then act on that socket/transfer only and not care about
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any other transfers. (The previous API always had to scan through all
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the existing transfers.)
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The idea is that curl_multi_socket() calls a given callback with information
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about what socket to wait for what action on, and the callback only gets
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called if the status of that socket has changed.
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In the API draft from before, we have a timeout argument on a per socket
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basis and we also allowed curl_multi_socket() to pass in an 'easy handle'
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instead of socket to allow libcurl to shortcut a lookup and work on the
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affected easy handle right away. Both these turned out to be bad ideas.
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The timeout argument was removed from the socket callback since after much
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thinking I came to the conclusion that we really don't want to handle
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timeouts on a per socket basis. We need it on a per transfer (easy handle)
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basis and thus we can't provide it in the callbacks in a nice way. Instead,
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we have to offer a curl_multi_timeout() that returns the largest amount of
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time we should wait before we call the "timeout action" of libcurl, to
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trigger the proper internal timeout action on the affected transfer. To get
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this to work, I added a struct to each easy handle in which we store an
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"expire time" (if any). The structs are then "splay sorted" so that we can
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add and remove times from the linked list and yet somewhat swiftly figure
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out 1 - how long time there is until the next timer expires and 2 - which
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timer (handle) should we take care of now. Of course, the upside of all this
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is that we get a curl_multi_timeout() that should also work with old-style
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applications that use curl_multi_perform().
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We also added a timer callback that makes libcurl call the application when
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the timeout value changes, and you set that with curl_multi_setopt().
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We created an internal "socket to easy handles" hash table that given
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a socket (file descriptor) return the easy handle that waits for action on
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that socket. This hash is made using the already existing hash code
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(previously only used for the DNS cache).
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To make libcurl able to report plain sockets in the socket callback, we had
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to re-organize the internals of the curl_multi_fdset() etc so that the
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conversion from sockets to fd_sets for that function is only done in the
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last step before the data is returned. I also had to extend c-ares to get a
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function that can return plain sockets, as that library too returned only
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fd_sets and that is no longer good enough. The changes done to c-ares have
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been committed and are available in the c-ares CVS repository destined to be
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included in the c-ares 1.3.1 release.
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We have done a test runs with up to 9000 connections (with a single active
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one). The curl_multi_socket() invoke then takes less than 10 microseconds in
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average (using the read-only-1-byte-at-a-time hack). We are now below the
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60 microseconds "per socket action" goal (the extra 50 is the time libevent
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needs).
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Status Right Now
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The curl_multi_socket() API is implemented according to how it is
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documented. We deem it ready to use.
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http://curl.haxx.se/libcurl/c/curl_multi_socket.html
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http://curl.haxx.se/libcurl/c/curl_multi_timeout.html
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http://curl.haxx.se/libcurl/c/curl_multi_setopt.html
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What is Left for the curl_multi_socket API
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Real world usage!
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