On Tue, 2 Dec 2014 06:57:50 -0500 Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx> wrote: > On Mon, 1 Dec 2014 19:38:19 -0500 > Trond Myklebust <trondmy@xxxxxxxxx> wrote: > > > On Mon, Dec 1, 2014 at 6:47 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > On Fri, Nov 21, 2014 at 02:19:30PM -0500, Jeff Layton wrote: > > >> Testing has shown that the pool->sp_lock can be a bottleneck on a busy > > >> server. Every time data is received on a socket, the server must take > > >> that lock in order to dequeue a thread from the sp_threads list. > > >> > > >> Address this problem by eliminating the sp_threads list (which contains > > >> threads that are currently idle) and replacing it with a RQ_BUSY flag in > > >> svc_rqst. This allows us to walk the sp_all_threads list under the > > >> rcu_read_lock and find a suitable thread for the xprt by doing a > > >> test_and_set_bit. > > >> > > >> Note that we do still have a potential atomicity problem however with > > >> this approach. We don't want svc_xprt_do_enqueue to set the > > >> rqst->rq_xprt pointer unless a test_and_set_bit of RQ_BUSY returned > > >> negative (which indicates that the thread was idle). But, by the time we > > >> check that, the big could be flipped by a waking thread. > > > > > > (Nits: replacing "negative" by "zero" and "big" by "bit".) > > > > Sorry, hit send too quickly... Thanks for fixing those. > > >> To address this, we acquire a new per-rqst spinlock (rq_lock) and take > > >> that before doing the test_and_set_bit. If that returns false, then we > > >> can set rq_xprt and drop the spinlock. Then, when the thread wakes up, > > >> it must set the bit under the same spinlock and can trust that if it was > > >> already set then the rq_xprt is also properly set. > > >> > > >> With this scheme, the case where we have an idle thread no longer needs > > >> to take the highly contended pool->sp_lock at all, and that removes the > > >> bottleneck. > > >> > > >> That still leaves one issue: What of the case where we walk the whole > > >> sp_all_threads list and don't find an idle thread? Because the search is > > >> lockess, it's possible for the queueing to race with a thread that is > > >> going to sleep. To address that, we queue the xprt and then search again. > > >> > > >> If we find an idle thread at that point, we can't attach the xprt to it > > >> directly since that might race with a different thread waking up and > > >> finding it. All we can do is wake the idle thread back up and let it > > >> attempt to find the now-queued xprt. > > > > > > I find it hard to think about how we expect this to affect performance. > > > So it comes down to the observed results, I guess, but just trying to > > > get an idea: > > > > > > - this eliminates sp_lock. I think the original idea here was > > > that if interrupts could be routed correctly then there > > > shouldn't normally be cross-cpu contention on this lock. Do > > > we understand why that didn't pan out? Is hardware capable of > > > doing this really rare, or is it just too hard to configure it > > > correctly? > > > > One problem is that a 1MB incoming write will generate a lot of > > interrupts. While that is not so noticeable on a 1GigE network, it is > > on a 40GigE network. The other thing you should note is that this > > workload was generated with ~100 clients pounding on that server, so > > there are a fair amount of TCP connections to service in parallel. > > Playing with the interrupt routing doesn't necessarily help you so > > much when all those connections are hot. > > In principle though, the percpu pool_mode should have alleviated the contention on the sp_lock. When an interrupt comes in, the xprt gets queued to its pool. If there is a pool for each cpu then there should be no sp_lock contention. The pernode pool mode might also have alleviated the lock contention to a lesser degree in a NUMA configuration. Do we understand why that didn't help? In any case, I think that doing this with RCU is still preferable. We're walking a very short list, so doing it lockless is still a good idea to improve performance without needing to use the percpu pool_mode. > > > - instead we're walking the list of all threads looking for an > > > idle one. I suppose that's tpyically not more than a few > > > hundred. Does this being fast depend on the fact that that > > > list is almost never changed? Should we be rearranging > > > svc_rqst so frequently-written fields aren't nearby? > > > > Given a 64-byte cache line, that is 8 pointers worth on a 64-bit processor. > > > > - rq_all, rq_server, rq_pool, rq_task don't ever change, so perhaps > > shove them together into the same cacheline? > > > > - rq_xprt does get set often until we have a full RPC request worth of > > data, so perhaps consider moving that. > > > > - OTOH, rq_addr, rq_addrlen, rq_daddr, rq_daddrlen are only set once > > we have a full RPC to process, and then keep their values until that > > RPC call is finished. That doesn't look too bad. > > That sounds reasonable to me. -- Jeff Layton <jlayton@xxxxxxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
