support rtree preheat

[hy05190134]

Issue

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we found a lot of requests timeout when osrm is starting, but after a while the rt decrease, so we want to resolve the problem, and we found the reason is the preheat of rtree, because the leaf node of rtree is stored in the .fileIndex file and
it use mmap so it will produce disk I/O for preheat

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[danpat]

Hmm. Why not run a simple cat map.fileIndex > /dev/null operation before launching osrm-routed? That would have the same effect of loading all the files pages into the page cache....

[hy05190134]

Hmm. Why not run a simple cat map.fileIndex > /dev/null operation before launching osrm-routed? That would have the same effect of loading all the files pages into the page cache....

I don't think it will produce the same effect, cat file just preheat the cache of disk but will not preheat the mmap page cache and I just want others do not sense the process when they first use the osrm-routed and encounter the same problem when started

[hy05190134]

@danpat another question, why the desinger of osrm just leave the .fileIndex using mmap