thread taskq / unp_gc() using 100% cpu and stalling unix socket IPC

Alfred Perlstein bright at
Wed Nov 14 03:29:50 UTC 2012

On 11/13/12 4:41 PM, Markus Gebert wrote:
> On 13.11.2012, at 19:30, Markus Gebert <markus.gebert at> wrote:
>> To me it looks like the unix socket GC is triggered way too often and/or running too long, which uses cpu and worse, causes a lot of contention around the unp_list_lock which in turn causes delays for all processes relaying on unix sockets for IPC.
>> I don't know why the unp_gc() is called so often and what's triggering this.
> I have a guess now. Dovecot and relayd both use unix sockets heavily. According to dtrace uipc_detach() gets called quite often by dovecot closing unix sockets. Each time uipc_detach() is called unp_gc_task is taskqueue_enqueue()d if fds are inflight.
> in uipc_detach():
> 682		if (local_unp_rights)	
> 683			taskqueue_enqueue(taskqueue_thread, &unp_gc_task);
> We use relayd in a way that keeps the source address of the client when connecting to the backend server (transparent load balancing). This requires IP_BINDANY on the socket which cannot be set by unprivileged processes, so relayd sends the socket fd to the parent process just to set the socket option and send it back. This means an fd gets transferred twice for every new backend connection.
> So we have dovecot calling uipc_detach() often and relayd making it likely that fds are inflight (unp_rights > 0). With a certain amount of load this could cause unp_gc_task to be added to the thread taskq too often, slowing everything unix socket related down by holding global locks in unp_gc().
> I don't know if the slowdown can even cause a negative feedback loop at some point by inreasing the chance of fds being inflight. This would explain why sometimes the condition goes away by itself and sometimes requires intervention (taking load away for a moment).
> I'll look into a way to (dis)prove all this tomorrow. Ideas still welcome :-).

A couple of ideas:

1) convert the taskqueue to a callout, but only allow one to be queued 
at a time.  set the granularity.

2) I think you only need to actually run garbage collection on the 
off-chance that you pass unix file descriptors, otherwise you can get 
away with refcounting.  It's hard for me to express the exact logic 
needed for this though.  I think you would need some way to simply do 
refcounting until there was a unix socket descriptor in flight, then 
switch to gc.   Either that or make a sysctl that allows you 
administratively deny passing of unix descriptors and just use refcounting.

Or just use Adrian's hack. :)


More information about the freebsd-stable mailing list