113 lines
6.3 KiB
Plaintext
113 lines
6.3 KiB
Plaintext
Implementation of the curl_multi_socket API
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Most of the design decisions and debates about this new API have already
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been held on the curl-library mailing list a long time ago so I had a basic
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idea on what approach to use. 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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The easy handle argument was removed fom the curl_multi_socket() function
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because having it there would require the application to do a socket to easy
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handle conversion on its own. I find it very unlikely that applications
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would want to do that and since libcurl would need such a lookup on its own
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anyway since we didn't want to force applications to do that translation
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code (it would be optional), it seemed like an unnecessary option.
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Instead I 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 be able to report plain sockets in the socket callback, I
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had 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 upcoming c-ares 1.3.1 release.
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The 'shiper' tool is the test application I wrote that uses the new
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curl_multi_socket() in its current state. It seems to be working and it uses
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the API as it is documented and supposed to work. It is still using
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select(), because I needed that during development (like until I had the
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socket hash implemented etc) and because I haven't yet learned how to use
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libevent or similar.
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The hiper/shiper tools are very simple and initiates lots of connections and
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have them running for the test period and then kills them all.
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Since I wasn't done with the implementation until early January I haven't
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had time to run very many measurements and checks, but I have done a few
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runs with up to a few hundred connections (with a single active one). The
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curl_multi_socket() invoke then takes 3-6 microseconds in average (using the
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read-only-1-byte-at-a-time hack). If this number does increase a lot when we
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add connections, it certainly matches my in my opinion very ambitious goal.
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We are now below the 60 microseconds "per socket action" goal. It is
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destined to be somewhat higher the more connections we have since the hash
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table gets more populated and the splay tree will grow etc.
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Some tests at 7000 and 9000 connections showed that the socket hash lookup
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is somewhat of a bottle neck. Its current implementation may be a bit too
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limiting. It simply has a fixed-size array, and on each entry in the array
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it has a linked list with entries. So the hash only checks which list to
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scan through. The code I had used so for used a list with merely 7 slots (as
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that is what the DNS hash uses) but with 7000 connections that would make an
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average of 1000 nodes in each list to run through. I upped that to 97 slots
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(I believe a prime is suitable) and noticed a significant speed increase. I
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need to reconsider the hash implementation or use a rather large default
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value like this. At 9000 connections I was still below 10us per call.
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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.
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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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1 - More measuring with more extreme number of connections
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2 - More testing with actual URLs and complete from start to end transfers.
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I'm quite sure we don't set expire times all over in the code properly, so
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there is bound to be some timeout bugs left.
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What it really takes is for me to commit the code and to make an official
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release with it so that we get people "out there" to help out testing it.
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