Messages in this thread | | | Subject | Re: [patch 2/2] sched: Scale the nohz_tracker logic by making it per NUMA node | From | Peter Zijlstra <> | Date | Mon, 14 Dec 2009 23:58:16 +0100 |
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On Mon, 2009-12-14 at 14:32 -0800, Pallipadi, Venkatesh wrote: > On Mon, 2009-12-14 at 14:21 -0800, Peter Zijlstra wrote: > > On Thu, 2009-12-10 at 17:27 -0800, venkatesh.pallipadi@intel.com wrote: > > > Having one idle CPU doing the rebalancing for all the idle CPUs in > > > nohz mode does not scale well with increasing number of cores and > > > sockets. Make the nohz_tracker per NUMA node. This results in multiple > > > idle load balancing happening at NUMA node level and idle load balancer > > > only does the rebalance domain among all the other nohz CPUs in that > > > NUMA node. > > > > > > This addresses the below problem with the current nohz ilb logic > > > * The lone balancer may end up spending a lot of time doing the > > > * balancing on > > > behalf of nohz CPUs, especially with increasing number of sockets and > > > cores in the platform. > > > > If the purpose is to keep sockets idle, doing things per node doesn't > > seem like a fine plan, since we're having nodes <= socket machines these > > days. > > The idea is to do idle balance only within the nodes. > Eg: 4 node (and 4 socket) system with each socket having 4 cores. > If there is a single active thread on such a system, say on socket 3. > Without this change we have 1 idle load balancer (which may be in socket > 0) which has periodic ticks and remaining 14 cores will be tickless. > But this one idle load balancer does load balance on behalf of itself + > 14 other idle cores. > > With the change proposed in this patch, we will have 3 completely idle > nodes/sockets. We will not do load balance on these cores at all.
That seems like a behavioural change, not balancing these 3 nodes at all could lead to overload scenarios on the one active node, right?
> Remaining one active socket will have one idle load balancer, which when > needed will do idle load balancing on behalf of itself + 2 other idle > cores in that socket.
> If there all sockets have atleast one busy core, then we may have more > than one idle load balancer, but each will only do idle load balance on > behalf of idle processors in its own node, so total idle load balance > will be same as now.
How about things like Magny-Cours which will have multiple nodes per socket, wouldn't that be best served by having the total socket idle, instead of just half of it?
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