Experiment with mmap in StaticRTree

[danpat]

We can possibly save the cost of a syscall in our StaticRtree here:

osrm-backend/include/util/static_rtree.hpp

Lines 445 to 460 in 908a9e9

	inline void LoadLeafFromDisk(const uint32_t leaf_id, LeafNode &result_node)
	{
	if (!leaves_stream.is_open())
	{
	leaves_stream.open(m_leaf_node_filename, std::ios::in | std::ios::binary);
	}
	if (!leaves_stream.good())
	{
	throw exception("Could not read from leaf file.");
	}
	const uint64_t seek_pos = sizeof(uint64_t) + leaf_id * sizeof(LeafNode);
	leaves_stream.seekg(seek_pos);
	BOOST_ASSERT_MSG(leaves_stream.good(), "Seeking to position in leaf file failed.");
	leaves_stream.read((char *)&result_node, sizeof(LeafNode));
	BOOST_ASSERT_MSG(leaves_stream.good(), "Reading from leaf file failed.");
	}

by using mmap to access the leaf node data.

This StackOverflow ticket: http://stackoverflow.com/a/5589622
talks about some of the tradeoffs. Because we do quite a lot of IO on each nearest-neighbor search, it's not clear to me whether we're a good candidate for mmap. We should measure this.

/cc @daniel-j-h

[daniel-j-h]

Hints

• file_mapping / mapped_region
• non-owning array_view

[TheMarex]

This is implemented as part of #2385

[TheMarex]

Landed.

[daniel-j-h]

Did you try setting hints such as madvise or pre-faulting pages? I think the Boost.IOStream's mmap features are very basic only and do not support this. Boost.Interprocess' mapped_region is more advanced in this regard. (Otherwise ifdef and syscalls)