Advise/Prefetch Wrapper

[johnpatek]

Hi @johnpatek I've been using mio for a few years to map a 400 GB file that I use as a circular file to simulate a 15-20TB file. Obviously it constantly gets over written but I also read/write randomly, sequentially write 1.5GB chunks. I have looked with envy at the linux madvise() function, large/huge memory pages but I'm basically left with what Windows has had since the 32 era. 64 bit has improved the memory map size but the API has remained pretty basic. The performance of my application is totally at the mercy of the Windows memory manager which is a black box although we have recently got PrefetchVirtualMemory(). Good luck

Originally posted by @spudwa in #15

As discussed in the above issue, it would be helpful to add a method to the mapping that allows Windows and Linux to use PrefetchVirtualMemory and madvise, respectively. There is a huge gap in what the APIs actually offer, but there's enough overlap in what people might use them for that this feature could be worthwhile. A quick and dirty outline of the provisional API could be the following:

enum class advice
{
    will_need
    dont_need,
    access_random,
    access_sequential
};

void advise(void *addr, size_t len, advice adv);

The flags access_* can be ignored for Windows, as PrefetchVirtualMemory is essentially brute force control over paging.

[johnpatek]

Some open questions:

• Would it be useful to have a second advise function that takes an offset from the base handle to determine the memory address being advised?
• PrefetchVirtualMemory allows address/length pairs to be submitted as a batch. Is this something worth exposing at the top level API, or is it just another Windows quirk that we can keep hidden in the abstraction?

[johnpatek]

@wissem01chiha please let me know if you have any thoughts about this issue.