We're running Nginx version 1.15.8 but we've been seeing similar issues with other versions too and on all of our servers that have a high number of vhosts.

The issue is that when you do an nginx reload it ends up using almost 2x the ram as it was previously. Here is a test I ran.
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21.3 MiB + 1.4 GiB = 1.4 GiB nginx (3)
21.3 MiB + 1.4 GiB = 1.4 GiB nginx (3)
484.2 MiB + 1.4 GiB = 1.9 GiB nginx (3)
588.1 MiB + 1.4 GiB = 2.0 GiB nginx (3)
720.3 MiB + 1.4 GiB = 2.1 GiB nginx (3)
1.4 GiB + 1.4 GiB = 2.8 GiB nginx (3)
18.0 MiB + 2.7 GiB = 2.7 GiB nginx (3)
20.8 MiB + 2.7 GiB = 2.7 GiB nginx (3)
20.8 MiB + 2.7 GiB = 2.7 GiB nginx (3)
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I expect the ram usage to increase while the reload is happening but after it's done shouldn't the ram usage go back to about the same level?

This issue is completely reproducible across all of our servers and if I do a full restart, ram usage goes back down to normal.

Any thoughts?