[ovs-dev] Threaded userspace datapath

[Ed Maste]

On 9 August 2012 11:56, Ben Pfaff <blp at nicira.com> wrote:

> I'm curious about the performance improvement.  You mentioned a 10x
> performance improvement.  How much did CPU usage increase as part of
> that?  We already have users who complain when CPU usage spikes to 100%;
> I'm not sure that users will be happy if CPU usage can spike to 1000%
> (!).

Giuseppe did that basic benchmarking on Linux with the threaded
userspace patch and may have observed CPU usage statistics during that
work.  I don't have current numbers on the FreeBSD port, but will get
some soon.  Performance improvements are very important on FreeBSD of
course because we have only the userspace datapath.

> Oh I see, it looks like there is only one thread that does all packet
> processing?  I had a notion that there would be many threads, for
> example on a per-datapath or per-port basis.  It's very impressive
> getting 10x improvement with only a single additional thread.

Eventually I think a thread per datapath may be the way to go, and I
think the model in this patch can be reasonably extended to
per-datapath.

> I looked at the repository and then at the diff, both very briefly.  The
> repository looks more like development notes than something mergeable;
> the diff was easier to read.

Yes, the repo started from a snapshot patch against 1.1.0pre2 and has
evolved over time.  To get to something mergeable I'd expect to roll
it all up, or refactor into two or three committable pieces.

> One early reaction to the code is that there are too many #ifdefs
> (mainly for locking and unlocking mutexes).  I think that we should be
> able to remove many of them with a few well-chosen macros.

Indeed; the original 1.1.0pre2 work included this:

/* We could use these macros instead of using #ifdef and #endif every time we
 * need to call the pthread_mutex_lock/unlock.
#ifdef THREADED
#define LOCK(mutex) pthread_mutex_lock(mutuex)
#define UNLOCK(mutex) pthread_mutex_unlock(mutex)
#else
#define LOCK(mutex)
#define UNLOCK(mutex)
#endif

but then never used those macros.  I can switch it over to this scheme
if this is an agreeable approach (or define macros for each individual
lock, e.g. TABLE_LOCK and PORT_LOCK).

> OVS recently got a library, lib/worker.c, for implementing code in a
> "worker process".  So far, we're using that in a single-worker-process
> model, mostly for asynchronous logging, but we could generalize it so
> that there could be a second worker process used for datapath
> implementation.  If this would achieve acceptable performance, then it
> would avoid the difficulties of threads.  I guess the question there is
> how much communication a process model would require versus a thread
> model.

Right now packets that had a flow lookup miss are the only data
flowing from the datapath thread to the main thread, so that should be
easily done in either a thread or process model.  In the other
direction the datapath thread needs access to the flow table and I
think this would be a bit more awkward to implement in a process
model.  (On the other hand, a userland datapath process may be a
decent analogue to the Linux kernel module.)

-Ed