diff options
| -rw-r--r-- | hiper/STATUS | 240 |
1 files changed, 240 insertions, 0 deletions
diff --git a/hiper/STATUS b/hiper/STATUS new file mode 100644 index 000000000..be1ff7fb9 --- /dev/null +++ b/hiper/STATUS @@ -0,0 +1,240 @@ +Date: January 5, 2006 +Author: Daniel Stenberg + + Status of project Hiper - high performance libcurl modifications + ================================================================ + +What is Hiper + + You won't find such a description in this document. See + http://curl.haxx.se/libcurl/hiper/ for further details. + +Live Progress Info + + During my work, I've posted occational updates on the curl-library mailing + list but more importantly done frequent updates of + http://curl.haxx.se/libcurl/hiper/schedule.html + +Schedule + + I took time off my regular job during Decemember 2005 and the first week of + January 2006 to work on hiper full-time. + +Step 1 - Measure the Existing Solution + + I started full-time work on project Hiper on December 1st 2005. I began by + putting together a test application that used the existing API to allow me + to properly and with accuracy measure execution and transfer speeds when + doing a large amount of transfers. + + I soon discovered that it was impossible to do any sensible measurements by + using live and actual URLs since the transfers were too unrelialble and + uncontrolled. I then enhanced the current HTTP server in the curl test suite + and made that support a large amount of transfers and some extra magic + "commands" that would make the server either just sit "idle" or "stream" + (continuously sending data in a never-ending stream). I then wrote up two + files using the curl test suite file format and by acessing the properly + formatted URLs on my localhost the HTTP server would either run "idle" or + run "stream". + + Having this working, I patched libcurl to always only recv() a single byte + off the network each time, just to make sure that the time spent on reading + data is constant and never very long. + + I adjusted the test application (actually called 'hiper') to create Y idle + transfers and Z stream transfers, had it run for N seconds and then quit and + produce a summary on stdout. Now I got very solid and repeatable results. I + started to run repeated tests and save the results when I ran into the + dreaded 1024 socket maximum limit. + + One side of the problem is that the fd_set type only allows 1024 file + descriptors (on my Linux), which I had to solve by simply making my own type + with room for more connections and do ugly typecasts in the code. The other + side of the problem is that user applications have a limit imposed by the + system on the maximum amount of file descriptors it can have open and I had + to work around that by writing a special tool that runs setuid root that + increases the limit, downgrades to a normal user again and then run the + command line of your choice. This second approach has to be used for both + 'hiper' and the test HTTP server. (You need to build the HTTP server with + CURL_SWS_FORK_ENABLED defined to have it do forks since it isn't desirable + to do so when running the normal curl tests.) + + Now I could run my test program without problems. I decided to run the tests + with 1 stream connection and a varying amount of idle ones. I did 1001, + 2001, 3001, 5001 and 9001 connections and measured how long select() and + curl_multi_perform() (including the curl_multi_fdset() call) would take in + average, over a period of 20 seconds. I ran each test 5-6 times and I used + the average time of all the runs. + + The times in number of microseconds: + + Connections multi_perform select + 1001 3504 951 + 2001 7606 1988 + 3001 11045 2715 + 5001 16406 4024 + 9001 32147 8030 + + Test system + CPU: Athlon XP 2800 + RAM: 1 GB + Linux: 2.6 + glibc: 2.3.5 + libcurl: 7.15.1 + + The only reason I stopped at 9001 connections is that my test machine ran + out of avaiable memory by then as I ran the test server on the same machine, + and I didn't want to risk the test result accuracy by having it start using + the swap during the tests. + + It means that at 9000 connections we spend 40ms for each socket action, even + when only one socket ever have action. + + With these 32000 microseconds curl_multi_perform() takes for 9000 + connections, it loops 18000 laps which makes less than 2 microseconds per + lap. (Of course counting time/laps is an oversimplification, but anyway.) + Hopefully we should achieve less than 10 microseconds for each call to + curl_multi_socket() for an active connection. + + The timing graph displayed on the libevent site (duplicated on the hiper + project page) suggests that libevent is pretty much fixed at 50 microseconds + (although I don't know what test box was used in their testing, we can + compare the select()-times from my tests and see that they are at least + resonably close). + + Summing up, the current ~40 ms spent at 9000 connections could then possibly + be lowered to something around 60 us! + +Step 2 - Implement 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. + + 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. The man pages for curl_multi_socket and curl_multi_timeout are + both committed to CVS and are available online for easy browsing: + + http://curl.haxx.se/libcurl/c/curl_multi_socket.html + http://curl.haxx.se/libcurl/c/curl_multi_timeout.html + + The hiper-5.patch I made available early morning January 5th, 2006 should + apply fine on a recent CVS checkout (at the time of this writing curl 7.15.1 + is the latest public curl release but the hiper patch does not apply fine on + that). + +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. + +What is Left for project Hiper + + 1 - Add HTTP pipelining support + + 2 - Add a zero (or at least close to zero) copy interface + + Neither of these points have been planned or detailed exactly how they will + be implemented. + +Roadmap Ahead + + I plan and hope to return to full-time hiper work later on this spring or + possibly summer to continue where I pause now. Of course some spare time + might also be spent until then to get us moving forward. |