Call resize to preallocate for memory does not work

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I have a large number of key/value pairs, say twenty million, to
insert into the dense_hash_map. I noticed that there are several
times of re-allocation happens in the process of inserting, and
the peak value of memory allocation is double than required.
In order to reduce the total times of re-allocation and gain more
efficiency, I'm considering about preallocation.

So I wrote:

dense_hash_map<k, v> m;
m.resize(20000000);
m.set_empty_key(0);
m.set_deleted_key(-1);

to do the preallocation, and after set_empty_key, I saw a big
700MBytes allocation in the memory, so far so good.

But, after first few(say 4 to 5) insert/erase calls, the big memory
block will be freed, and dense_hash_map repeats the re-allocation
until 1.2GBytes (peak value is 2.3GBytes).

What should i do please ?

Original issue reported on code.google.com by AmanJI...@gmail.com on 25 Jun 2014 at 5:43