path: root/Documentation/networking/filter.txt
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authorEric Dumazet <edumazet@google.com>2014-09-20 18:01:30 -0700
committerDavid S. Miller <davem@davemloft.net>2014-09-26 00:26:48 -0400
commit4a8e320c929991c9480a7b936512c57ea02d87b2 (patch)
treedf1c9960aa28db003cdea1558246a3e873c2f055 /Documentation/networking/filter.txt
parentMerge branch 'gso_send_check' (diff)
net: sched: use pinned timers
While using a MQ + NETEM setup, I had confirmation that the default timer migration ( /proc/sys/kernel/timer_migration ) is killing us. Installing this on a receiver side of a TCP_STREAM test, (NIC has 8 TX queues) : EST="est 1sec 4sec" for ETH in eth1 do tc qd del dev $ETH root 2>/dev/null tc qd add dev $ETH root handle 1: mq tc qd add dev $ETH parent 1:1 $EST netem limit 70000 delay 6ms tc qd add dev $ETH parent 1:2 $EST netem limit 70000 delay 8ms tc qd add dev $ETH parent 1:3 $EST netem limit 70000 delay 10ms tc qd add dev $ETH parent 1:4 $EST netem limit 70000 delay 12ms tc qd add dev $ETH parent 1:5 $EST netem limit 70000 delay 14ms tc qd add dev $ETH parent 1:6 $EST netem limit 70000 delay 16ms tc qd add dev $ETH parent 1:7 $EST netem limit 80000 delay 18ms tc qd add dev $ETH parent 1:8 $EST netem limit 90000 delay 20ms done We can see that timers get migrated into a single cpu, presumably idle at the time timers are set up. Then all qdisc dequeues run from this cpu and huge lock contention happens. This single cpu is stuck in softirq mode and cannot dequeue fast enough. 39.24% [kernel] [k] _raw_spin_lock 2.65% [kernel] [k] netem_enqueue 1.80% [kernel] [k] netem_dequeue 1.63% [kernel] [k] copy_user_enhanced_fast_string 1.45% [kernel] [k] _raw_spin_lock_bh By pinning qdisc timers on the cpu running the qdisc, we respect proper XPS setting and remove this lock contention. 5.84% [kernel] [k] netem_enqueue 4.83% [kernel] [k] _raw_spin_lock 2.92% [kernel] [k] copy_user_enhanced_fast_string Current Qdiscs that benefit from this change are : netem, cbq, fq, hfsc, tbf, htb. Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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