getmemoryinfo ("mode")
Returns an object containing information about memory usage.
1. "mode" determines what kind of information is returned. This argument is optional, the default mode is "stats".
- "stats" returns general statistics about memory usage in the daemon.
- "mallocinfo" returns an XML string describing low-level heap state (only available if compiled with glibc 2.10+).

Result (mode "stats"):
"locked": { (json object) Information about locked memory manager
"used": xxxxx, (numeric) Number of bytes used
"free": xxxxx, (numeric) Number of bytes available in current arenas
"total": xxxxxxx, (numeric) Total number of bytes managed
"locked": xxxxxx, (numeric) Amount of bytes that succeeded locking. If this number is smaller than total, locking pages failed at some point and key data could be swapped to disk.
"chunks_used": xxxxx, (numeric) Number allocated chunks
"chunks_free": xxxxx, (numeric) Number unused chunks

Result (mode "mallocinfo"):

> bitcoin-cli getmemoryinfo
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "getmemoryinfo", "params": [] }' -H 'content-type: text/plain;'
Don't trust. Verify. Docs extracted from src/rpc/misc.cpp#L311-L340.