This is a Sierra box, the system memory was suddenly out of usage. I have checked the system to find out that the keychain process was busy and used nearly all the memory.
I had to reboot the system, then i opened keychain to see the login items list is flashing, one unknown item in login keychain was keeping being deleted and inserted.
Want to know why, I opened activity monitor, then got to know assistantd was busy doing that. I killed the process, the keychain came to be in peace.
I think the process is used by Siri, as assistantd process came out again after I clicked the siri icon.
Even I do not use siri, I want to know why this could happen, and what is the real solution.