Messages in this thread | | | Date | Mon, 29 Jun 2009 10:16:15 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC patch] Use IPI_shortcut for lapic timer broadcast |
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* Luming Yu <luming.yu@gmail.com> wrote:
> On Mon, Jun 29, 2009 at 3:20 PM, Ingo Molnar<mingo@elte.hu> wrote: > > > > * Luming Yu <luming.yu@gmail.com> wrote: > > > >> Hello, > >> > >> We need to use IPI shortcut to send lapic timer broadcast > >> to avoid the latency of sending IPI one bye one on systems with many > >> logical processors when NO_HZ is disabled. > >> Without this patch,I have seen upstream kernel with RHEL 5 kernel > >> config boot hang . > > > > hm, that might be a valid optimization - but why does the lack of > > this optimization result in a hang? > > It is hang caused by kernel code for work around lapic-timer-stop > issue. With HZ=1000, and a lot of cpus (eg. 64 logical cpus), cpu > 0 will be busy working on send TIMER IPI instead of making > progress in boot (right after deep-C-state has been used).
that's a bit weird. With HZ=1000 we have 1000 usecs between each timer tick. Assuming a CPU sends to a lot of CPUs (64 logical CPUs) that means that each IPI takes more than ~15 microseconds to process. On what hardware/platform can this happen realistically?
Ingo
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