Date: Wed, 14 Nov 2012 01:41:04 +0100 From: Markus Gebert <markus.gebert@hostpoint.ch> To: freebsd-stable <freebsd-stable@freebsd.org> Subject: Re: thread taskq / unp_gc() using 100% cpu and stalling unix socket IPC Message-ID: <007F7A73-75F6-48A6-9C01-E7C179CDA48A@hostpoint.ch> In-Reply-To: <6908B498-6978-4995-B081-8D504ECB5C0A@hostpoint.ch> References: <6908B498-6978-4995-B081-8D504ECB5C0A@hostpoint.ch>
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On 13.11.2012, at 19:30, Markus Gebert <markus.gebert@hostpoint.ch> = 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. >=20 > 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)=09 683 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 :-). Markus
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