On Sun, Oct 26, 2014 at 2:23 PM, Nilay Vaish <[email protected]> wrote:
> > Marc Orr asked me the same question last year. I am pasting the examples > I gave him: > > a. the data in the message is stale, but the sender does not know about > it. Take a look at the MESI CMP directory protocol. In the case when an L1 > controller (A) sends a PUTX to the L2 controller, it is possible that the > L2 controller has already transferred the ownership to some L1 controller > (B). In this case, it is possible that there are two message buffers that > contain messages from A and B to the L2 controller, but it is message from > B which has the 'right' data. > Interesting. I can see how this technically could be a problem, but it seems like a pretty unlikely corner case. Have you seen it happen in practice, and if so, what was the functional read for? I suppose I just have a hard time imagining an actual program that has a lot of contention on a block that ends up being used as a parameter to a system call. I guess it could happen with a syscall that's specifically for synchronization, like futex. > > b. no data is present in the message and the receiver will infer that the > data it has is correct since the message did not have any data. > This seems like it should pretty easy to fix... if you're querying the message to see if it has relevant data, then if the address matches but there is no data, you should just return false. I'd think there'd be a protocol-independent way to determine that a message has no data. It's similar to the idea that you have to check the valid bit in the cache, you can't just look for a tag match. Steve _______________________________________________ gem5-dev mailing list [email protected] http://m5sim.org/mailman/listinfo/gem5-dev
