Messages in this thread | | | Date | Fri, 19 Mar 2004 22:52:03 +0530 | From | Dipankar Sarma <> | Subject | Re: CONFIG_PREEMPT and server workloads |
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On Thu, Mar 18, 2004 at 10:10:06PM -0800, Andrew Morton wrote: > The worst-case latency is during umount, fs/inode.c:invalidate_list() when > the filesystem has a zillion inodes in icache. Measured 250 milliseconds > on a 256MB 2.7GHz P4 here. OK, so don't do that. > > The unavoidable worst case is in the RCU callbacks for dcache shrinkage - > I've seen 25 millisecond holdoffs on the above machine during filesystem > stresstests when RCU is freeing a huge number of dentries in softirq > context.
What filesystem stresstest was that ?
> > This if Hard To Fix. Dipankar spent quite some time looking into it and > had patches, but I lost track of where they're at.
And I am still working on this on a larger scope/scale. Yes, I have a patch that hands over the rcu callbacks to a per-cpu kernel thread reducing the softirq time. However this is not really a solution to the overall problem, IMO. I am collecting some instrumentation data to understand softirq/rcu behavior during heavy loads and ways to counter long running softirqs.
Latency isn't the only issue. DoS on route cache is another issue that needs to be addressed. I have been experimenting with Robert Olsson's router test and should have some more results out soon.
Thanks Dipankar - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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