Messages in this thread | | | Date | Wed, 2 Sep 2009 18:24:51 +0200 | From | Nick Piggin <> | Subject | Re: tree rcu: call_rcu scalability problem? |
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On Wed, Sep 02, 2009 at 08:19:27AM -0700, Paul E. McKenney wrote: > On Wed, Sep 02, 2009 at 02:27:56PM +0200, Nick Piggin wrote: > > On Wed, Sep 02, 2009 at 11:48:35AM +0200, Nick Piggin wrote: > > > Hi Paul, > > > > > > I'm testing out scalability of some vfs code paths, and I'm seeing > > > a problem with call_rcu. This is a 2s8c opteron system, so nothing > > > crazy. > > > > > > I'll show you the profile results for 1-8 threads: > > > > > > 1: > > > 29768 total 0.0076 > > > 15550 default_idle 48.5938 > > > 1340 __d_lookup 3.6413 > > > 954 __link_path_walk 0.2559 > > > 816 system_call_after_swapgs 8.0792 > > > 680 kmem_cache_alloc 1.4167 > > > 669 dput 1.1946 > > > 591 __call_rcu 2.0521 > > > > > > 2: > > > 56733 total 0.0145 > > > 20074 default_idle 62.7313 > > > 3075 __call_rcu 10.6771 > > > 2650 __d_lookup 7.2011 > > > 2019 dput 3.6054 > > > > > > 4: > > > 98889 total 0.0253 > > > 21759 default_idle 67.9969 > > > 10994 __call_rcu 38.1736 > > > 5185 __d_lookup 14.0897 > > > 4475 dput 7.9911 > > Four threads runs on one socket but 8 threads runs on two sockets, > I take it?
Yes.
> > > 8: > > > 170391 total 0.0437 > > > 31815 __call_rcu 110.4688 > > > 12958 dput 23.1393 > > > 10417 __d_lookup 28.3071 > > > > > > Of course there are other scalability factors involved too, but > > > __call_rcu is taking 54 times more CPU to do 8 times the amount > > > of work from 1-8 threads, or a factor of 6.7 slowdown. > > > > > > This is with tree RCU. > > > > It seems like nearly 2/3 of the cost is here: > > /* Add the callback to our list. */ > > *rdp->nxttail[RCU_NEXT_TAIL] = head; <<< > > rdp->nxttail[RCU_NEXT_TAIL] = &head->next; > > Hmmm... That certainly is not the first list of code in call_rcu() that > would come to mind...
It's weird. I *think* I read the asm right, but oprofile maybe is not attributing the cost to the right instruction.
> > In loading the pointer to the next tail pointer. If I'm reading the profile > > correctly. Can't see why that should be a probem though... > > The usual diagnosis would be false sharing.
Hmm that's possible yes.
> Hmmm... What is the workload? CPU-bound? If CONFIG_PREEMPT=n, I might > expect interference from force_quiescent_state(), except that it should > run only every few clock ticks. So this seems quite unlikely.
It's CPU bound and preempt=y.
Workload is just 8 processes running a loop of close(open("file$i")) as I said though you probably won't be able to reproduce it on a vanilla kernel.
> Could you please try padding the beginning and end of struct rcu_data > with a few hundred bytes and rerunning? Just in case there is a shared > per-CPU variable either before or after rcu_data in your memory layout?
OK I'll try that.
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