As the guy who added thread support to memcached, I feel qualified to
answer this one:
What libevent doesn't support is sharing a libevent instance across
threads. It works just fine to use libevent in a multithreaded process
where only one thread is making libevent calls.
What also works, and this is what I did in memcached, is to use multiple
instances of libevent. Each call to event_init() will give you back an
"event base," which you can think of as a handle to a libevent instance.
(That's documented in the current manual page, but NOT in the older
manpage on the libevent home page.) You can have multiple threads each
with its own event base and everything works fine, though you probably
don't want to install signal handlers on more than one thread.
In the case of memcached, I allocate one event base per thread at
startup time. One thread handles the TCP listen socket; when a new
request comes in, it calls accept() then hands the file descriptor to
another thread to handle from that point on -- that is, each client is
bound to a particular thread. All you have to do is call
event_base_set() after you call event_set() and before you call event_add().
Unfortunately, you pretty much have to use pipe() to communicate between
libevent threads, That's a limitation, and honestly it's a pretty big
one: it makes a master/worker thread architecture, where one thread
handles all the I/O, much less efficient than you'd like. My initial
implementation in memcached used an architecture like that and it chewed
lots of CPU time. That's not really libevent's fault -- no UNIX-ish
system I'm aware of has an equivalent to the Windows
WaitForMultipleObjects API that allows you to wake up on semaphores /
condition variables and on input from the network. Without that, any
solution is going to end up using pipes (or maybe signals, which have
their own set of issues in a multithreaded context) to wake up the
libevent threads.
-Steve
Glen Hein wrote:
Hello,
I am doing research for a project that may follow an event-driven
multi-threaded model with heavy disk and network I/O. The platform
will be FreeBSD and Linux. Overall, we are seeking a C10K solution.
I started reading through the libevent source and noticed several
comments about not being thread-safe. However, I have seen references
to other projects that are multi-threaded and use libevent. I need to
explore libevent and determine if it is viable for my project. Have
there been discussions on threading issues in the mailing list
archive? Is there another source of information on threads and
libevent?
Thanks,
Glen Hein
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