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Whats so difficult about ISSU

On Fri, Nov 9, 2012 at 4:00 PM, Saku Ytti <saku at ytti.fi> wrote:
>> For IOS-XE we have Linux in charge of the scheduler with a
>> multi-threaded IOSd process responsible for the control plane.  I'm
> I'm sceptical if this means there isn't normal IOS run-to-completion
> scheduler, certainly not all ios processes are separate threads to linux
> kernel? But I guess this is moving target. Would be interesting to hear how
> many threads, what are threads relative priorities, what runs in each
> thread etc.
> But anyhow just to hear it is threaded, is good news. Does this mean, IOSd
> can capitalize on multiple cores? (Something JunOS cannot do today)

I do not believe that the linux scheduler is run to completion, but to
be honest I'm not 100% certain. I know a big reason for IOS-XE was to
be able to operate in multicore environments. From a high level you
have IOSd as a process with each traditional process (BGP, OSPF, IP
Input) as a thread within IOSd. Overall IOS-XE is Linux managing a few
processes: IOSd, FMan-RP, CMan-RP (and a few others) FMan deals with
adjacencies and CMan deals with modules/cards and IOSd all the
interesting stuff. Since Linux is the piece actually running the show
IOS-XE gets all the memory management and scheduling benefits that
linux has.