[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Is continuous replication of state possible?
On Mon, 2005-01-10 at 01:21 +0900, aq wrote: > Hello, > > I am sorry for a possible stupid question, but may anybody please > explain to me what this "deterministic" means? Deterministic means that the outcome is a function of nothing more than the starting state and the input so if you start with the same starting state and apply the same input you are guaranteed to end up with the same outcome every time. So, deterministic code works with the replicated state machine approach because the replicas start off in the same state and have the same input applied and the same execution happens on all the replicas and they end up in the same state and so remain replicas of one another. If you try to execute non-deterministic code in the context of the replicated state machine approach then the replicas might start off in the same state and receive the same input but the different results from executing the non-deterministic code would cause them to diverge. Once the replicas have diverged they are no longer good as backups of one another. Examples of operations that would ordinarily be non-deterministic: Asking the CPU for a random number---All of the replicas start off in the same state with the program counter pointing to an instruction that asks for a random number; all of the replicas receive the same input: a command instructing them to execute one instruction; all of the replicas ask the CPU for a random number. All the replicas get a different random number from their CPU and the subsequent execution diverges. Taking an interrupt from a peripheral device---typically this will only happen on one of the nodes. If it was fed directly to a replica then that replica would immediately diverge from the others. -- Harry Butterworth <harry@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> ------------------------------------------------------- The SF.Net email is sponsored by: Beat the post-holiday blues Get a FREE limited edition SourceForge.net t-shirt from ThinkGeek. It's fun and FREE -- well, almost....http://www.thinkgeek.com/sfshirt _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.sourceforge.net/lists/listinfo/xen-devel
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