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[rsx]

Is this the only thing running within the VM?

Yes (except one small perl script).

What’s the result of cat /proc/meminfo | grep MemFree ?

MemFree: 758840 kB

But more than 3 GB is for cache.

Did you follow the Standard Install and run discourse-setup?

The latter without any changes.

What are db_shared_buffers, unicorn_workers and db_work_mem in your app.yml?

db_shared_buffers: "1024MB"
no entry for unicorn_workers
#db_work_mem: "40MB"