c++: how to optimize IO?

Question

I am working on a mathematical problem that has the advantage of being able to "pre-compute" about half of the problem, save this information to file, and then reuse it many times to compute various 'instances' of my problem. The difficulty is that uploading all of this information in order to solve the actual problem is a major bottleneck.

More specifically: I can pre-compute a huge amount of information - tons of probabilities (long double), a ton of std::map<int,int>, and much more - and save all this stuff to disk (several Gb).

The second half of my program accepts an input argument D. For each D, I need to perform a great many computations that involve a combination of the pre-computed data (from file), and some other data that are specific to D (so that the problem is different for each D).

Sometimes I will need to pick out certain pieces of pre-computed information from the files. Other times, I will need to upload every piece of data from a (large) file.

Are there any strategies for making the IO faster?

I already have the program parallelized (MPI, via boost::mpi) for other reasons, but regardless, accessing files on the disk is making my compute time unbearable.

Any strategies or optimizations?

Currently I am doing everything with cstdio, i.e. no iostream. Will that make a big difference?

Answer 1

Certainly the fastest (but the fragilest) solution would be to mmap the data to a fixed address. Slap it all in one big struct, and instantiate the std:::map with an allocator which will allocate in a block attached to the end of the struct. It's not simple, but it will be fast; one call to mmap, and the data is in your (virtual) memory. And because you're forcing the address in mmap, you can even store the pointers, etc.

As mentioned above, in addition to requiring a fair amount of work, it's fragile. Recompile your application, and the targeted address might not be available, or the layout might be different, or whatever. But since it's really just an optimization, this might not be an issue; anytime a compatibility issue arises, just drop the old file and start over. It will make the first run after a change which breaks compatibility extremely slow, but if you don't break compatibility too often...