In mongod's slow log. I got so many lines like below:
Mon Apr 10 11:05:01.426 I COMMAND [ftdc] serverStatus was very slow:

{ after basic: 0, after asserts: 0, after connections: 0, after extra_info: 1410, after globalLock: 1410, after locks: 1410, after network: 1410, after opcounters: 1410, after opcountersRepl: 1410, after repl: 1410, after storageEngine: 1410, after tcmalloc: 1410, after wiredTiger: 1410, at end: 1420 }

there are so many lines like above.
from the ftdc flag, I know it is generated from the diagonal thread, but this is not important.

What is important is that time all grows after getting extra_info. Which I found is to run mallinfo() on linux after digging into the code.

I doubt that mallinfo may be slow in some situations because all the slow logs I got is caused by the time-cost after extra_info.