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Daniel Stenberg 2006-10-29 09:18:32 +00:00
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lib/README.multi_socket
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@ -1,8 +1,6 @@
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:
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
@ -38,62 +36,33 @@ Implementation of the curl_multi_socket API
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.
We also added a timer callback that makes libcurl call the application when
the timeout value changes, and you set that with curl_multi_setopt().
Instead I created an internal "socket to easy handles" hash table that given
We 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
To make libcurl able to report plain sockets in the socket callback, we 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.
included in the 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.
We have done a test runs with up to 9000 connections (with a single active
one). The curl_multi_socket() invoke then takes less than 10 microseconds in
average (using the read-only-1-byte-at-a-time hack). We are now below the
60 microseconds "per socket action" goal (the extra 50 is the time libevent
needs).
Status Right Now
The curl_multi_socket() API is implemented according to how it is
documented.
documented. We deem it ready to use.
http://curl.haxx.se/libcurl/c/curl_multi_socket.html
http://curl.haxx.se/libcurl/c/curl_multi_timeout.html
@ -101,12 +70,4 @@ Status Right Now
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.
Real world usage!