On Fri, Apr 10, 2020, 4:12 AM Heiser, Gernot (Data61, Kensington NSW) < gernot.hei...@data61.csiro.au> wrote:
> >> If I have a manycore machine, what is the best way to migrate a thread > from > >> one core to another? Similarly, how can I send a capability (such as to > >> memory)? Cryptography is an option for the second one, but seems > inelegant > >> and inefficient. > > > I suspect Curtis didn’t quite give the answer the question Demi was after. > > Yes, in an SMP configuration, nothing special is needed. (And you don’t > need nor should use cross-core IPC – that feature may go away). > > But if you are running a (clustered) multikernel and want to run a single > system image (SSI) across clusters (eg to support a virtualised NUMA-aware > Linux on top) then it’s no different from a distributed system (except that > cross-cluster communication should be reliable, so a fair amount of > complexity in the protocols goes away. > > Basically you have a bunch of separate systems, and the SSI is a > higher-level software abstraction. Eg to have an abstraction of an object > that is valid across the whole system, each node in has to provide its own > copy of that object, and you need protocols to keep them coherent. This > applies whether the object looks like an seL4 kernel object (address space, > endpoint) or a Linux-style higher-level abstraction such as a file. > > And no, you can’t send caps across such a cluster, an seL4 cap has no > meaning outside its kernel. You could have a similar cap-protected object > abstraction, but both the distributed object and its caps will map to > objects of the local kernel (and the abstraction layer would be responsible > keeping the various local representations coherent). > > If this sounds crazy and complex, then that’s what it is. It just means > that the seL4 abstractions, which are meant to be minimal wrappers around > hardware, are just not at the right abstraction level for such a manycore > system. Which is not surprising: it’s what you get if you pretend that a > system where a message takes 1,000 cycle to get from one end to the other > is just a simple multicore. Such a manycore is effectively a distributed > system and should be treated as one. > If I may ask, why is this? What prevents the SMP kernel from scaling to manycore systems? Is it the complexity of verifying fine-grained locking, the cost of atomic operations, or something else I missed? Gernot > > _______________________________________________ > Devel mailing list > Devel@sel4.systems > https://sel4.systems/lists/listinfo/devel > _______________________________________________ Devel mailing list Devel@sel4.systems https://sel4.systems/lists/listinfo/devel
