On Wed, Apr 02, 2008 at 10:15:59PM +0200, Cezary Rzewuski wrote: > Hi, > I'd like to ask if sending http requests with libevent is carried out in > separate thread or is the library > single-threaded? I want to use the library in a program which will visit > many URL and download > it's content. Is it good idea to use libevent or the classic solution > with creating a separate thread per URL > request will be much more efficient solution?
It depends. What you describe is not nearly enough informatio to even give a suggestion. One thread per URL normally is a very poor choice (just as a matter of runtime efficiency), unless each URL causes you to do a lot of disk I/O, or if each URL causes you to do CPU intensive operations, like decode compressed audio/video. In each of those two situations, the process context switching costs are diminished relative to the type of work being done. Basically, the idea is that if your thread will block on an operation--CPU or I/O--but another thread running in parallel (not merely concurrently) could utilize additional resources, you want to multi-thread. If your application is merely moving bytes (say, as a proxy), usually a single thread is enough; you can multiplex non-blocking network operations on a single thread. In that sense, you're "switching contexts" in the application, and not the kernel. This reduces the workload, because context switching in the kernel is usually more expensive., OTOH, copying data in itself can be CPU intensive. If you read into a buffer from one socket, you might evict previous data you read in earlier. If you then try to re-read and/or copy that previous data over to another buffer later, the process will block as the data is fetched from RAM. If your proxy is even on a 100Mb connection, depending on how you process the data, you most definitely will need multiple threads. That's because 100Mb of network data could ballon to 5x or 10x that mount of byte shuffling. Of course, depending on how the L1, L2 and L3 caches are shared, it might not actually make much of a difference. It all depends! Of course, you can always use an event-oriented model within each particular thread. Or spread event delivery and processing across multiple threads. Given that you seem new to this (or at least new to the particular problem you're trying to solve), your best bet is to use a single thread using libevent, or go totally multi-threaded without libevent. In 90% of the circumstances one of those options (though not both) are as near to optimal as you'll get, and you don't need to the headaches of any additional complexity. > I saw that libevent was used in spybye, which is kind of similar what I > want to do. I was wondering if spybye were more efficient with requests > served in separate threads instead of using libevent (I don't say that > it's not efficient, just theoretically). I'm not sure, maybe its most efficient using _both_. But I suspect it probably just uses libevent in a single thread. Note, there are other ways to use threads. You could use one thread using libevent to handle all your queries and network I/O. Then you could use a separate thread worker pool to, for instance, run ClamAV on the data. This works well if you can isolate your CPU intensive work outside the mundane network I/O parts. If your application is overall CPU bound, and latency of particular requests isn't of primary concern, then it doesn't matter that libevent is running in a single thread. All your CPUs are doing work, just not the same types of work. _______________________________________________ Libevent-users mailing list Libevent-users@monkey.org http://monkeymail.org/mailman/listinfo/libevent-users
