Hello, I'm wondering whether we could introduce a /dev/same device to allow costless huge virtual memory. The use case is the simulation of the execution of a big irregular HPC application, to provision memory usage, cpu time, etc. We know how much time each computation loop takes, and it's easy to replace them with a mere accounting. We'd however like to avoid having to revamp the rest of the code, which does allocation/memcpys/etc., by just replacing the allocation calls with virtual allocations, i.e. allocations which return addresses of buffers that one can read/write, but the values you read are not necessarily what you wrote, i.e. the data is not actually properly stored (since we don't do the actual computations that's not a problem). The way we currently do this is by some folding: we map the same normal file several times contiguously to form the virtual allocation. By using a small 1MiB file, this limits memory consumption to 1MiB plus the page table (and fits the dumb data in a typical cache). This however creates one VMA per file mapping, we get limited by the 65535 VMA limit, and VMA lookup becomes slow. The way I could see is to have a /dev/same device: when you open it, it allocates one page. When you mmap it, it maps the same page over the whole resulting single VMA. This is a quite specific use case, but it seems to be easy to implement, and it seems to me that it could be integrated mainline. Actually I was thinking that /dev/null itself could be providing that service? (currently it returns ENODEV) What do people think? Is there perhaps another solution to achieve this that I didn't think about? Samuel -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
