Thank you for Your suggestions. I've just finished the implementation.
I used the approach of libevent as HTTP server and threads working
on downloaded content (they are performing some statistical computation on
downloaded javascripts). It looks to work efficiently.
It's probably not the right group, but you says that switching between
threads
is expensive. However, I've read somewhere (it was probably "Advanced linux
programming" by Alex Samuel) that creating a new thread is nearly as fast as
calling a function. Does it mean, that switching between threads is
slower than
creating a new thread?
Once more - thanks for comprehensive answer.
William Ahern wrote:
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.
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