Re: [patch] latency tracer, 2.6.15-rc7

On Fri, 2005-12-30 at 17:02 -0800, Linus Torvalds wrote:
> 
> On Fri, 30 Dec 2005, Lee Revell wrote:
> > 
> > It seems that the networking code's use of RCU can cause 10ms+
> > latencies:
> 
> Hmm. Is there a big jump at the 10ms mark? Do you have a 100Hz timer 
> source? 
> 
> A latency jump at 10ms would tend to indicate a missed wakeup that 
> was "picked up" by the next timer tick.

No there are no large jumps, it really seems that this was the network
code causing an RCU callback to drop ~2K routes at once.  Specifically
RCU invokes dst_rcu_free 2085 times in a single batch
(call_rcu_bh(&rt->u.dst.rcu_head, dst_rcu_free) is only called from
rt_free() and rt_drop()).

I have found that many of the paths in the network stack that can cause
high latencies can be tuned via sysctls (for example
net.ipv4.route.gc_thresh); this one may be the same.

Lee

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