Threaded interrupts and preemption

Bryan Wu bryan.wu at
Tue Apr 13 03:28:15 UTC 2010

On 04/12/2010 04:24 PM, Chase Douglas wrote:
> Andy
> Here at ELC there's a big push behind RT and the underlying tech like
> threaded ISRs and varying levels of preemption. In a talk about
> threaded ISRs, the presenter had a graph of some latency measurement
> (I can't remember what exactly). The graph was rather stunning. It
> showed RHEL 5 as having really high latency average and very large
> distribution. It then had a fully preemptible kernel, and that showed
> great average latency, but a reasonable distribution of latency.
> Finally was threaded ISRs from the RT patch set, which averaged very
> slightly above the full preemption, but EXTREMELY predictable latency.
> I noticed that our Lucid kernel only has voluntary preemption. Have we
> looked into full preemption?

Yeah, it is better we can discuss these kinds of options in UDS-M.

> Perhaps we can get better desktop
> responsiveness just by going that far. In particular I am interested
> in trying out the RT patchset with threaded ISRs to see what effect
> that has as well, though we wouldn't ship it as a supported kernel
> configuration. Perhaps we would find that our responsiveness issues
> with high I/O are due to ISR latencies?
> In the RT patch all ISRs are threaded by default unless they
> specifically set it up otherwise. I think it would be fun to have an
> RT completely preemptible kernel just so we can try it out and have a
> comparison. We would gain an insight as to whether a responsiveness
> issue is scheduler algorithm based or latency based.
> -- Chase

Bryan Wu <bryan.wu at>
Kernel Developer    +86.138-1617-6545 Mobile
Ubuntu Kernel Team | Hardware Enablement Team
Canonical Ltd.
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