Messages in this thread | | | Date | Mon, 29 Feb 2016 17:28:35 +0100 | From | Samuel Thibault <> | Subject | Costless huge virtual memory? /dev/same, /dev/null? |
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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
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