Memory allocated by new mapnik.Image() doesn't seem to be freed by GC

[zdila]

Our server is regularily killed by kernel because it eats all the RAM. It seems to be caused by not freeing unreferenced images created by new mapnik.Image(...);

Testcase:

for (let i = 0; i < 1000000; i++) {
  new mapnik.Image(256, 256);
}

[zdila]

I am not sure if this issue affects only image or other types, for example mapnik.Map, mapnik.Color. Also if image.resize() returns new image then maybe it is also never freed.

[springmeyer]

@zdila can you try the v4.5.1 release to see if it improves your memory usage? Based on my testing, it should.

[zdila]

@springmeyer I can reproduce the testcase also in version 4.5.2 (eats 16GB ram in 4 seconds and then starts to swap).

[artemp]

@zdila - This is not a memory leak but rather design issue/feature

mapnik.Image is allocating external memory : https://github.com/mapnik/node-mapnik/blob/master/src/mapnik_image.cpp#L195 but GC is not aware of this. The way to inform GC about these allocations is to call Napi::MemoryManagement::AdjustExternalMemory(env, size); when image allocated and Napi::MemoryManagement::AdjustExternalMemory(env, -size); when finaliser is called. Those calls don't come for free, though.

It's more efficient to do something like

var mapnik = require('../');
for (let i = 0; i < 1000000; i++) {
  new mapnik.Image(256, 256);
  if (i % 1000 == 0)
  {
    global.gc();
  }
}

and not call Napi::MemoryManagement::AdjustExternalMemory for ever new mapnik.Image(...) . I guess this was behind original motivation for this implementation.
/cc @springmeyer

[zdila]

Thanks for the explanation. To prevent memory leaks we already implemented reusing mapnik.Image instances in our project.