GNOME System Monitor

Very customizable (choose among 24 key process statistics, drag-n-drop to rearrange columns, one-click column sort). Includes per-process I/O rates, which many tools don't bother with. Can search for open files as well.

Nice, only lacks writing/reading output and temperatures.

Around 60 MB of RSS for 3.36.

"Resources" tab can work with simple apps and 40% CPU without significant
overhead.

"Processes" tab can update at 1 Hz.
"Resources" tab can update at 4 Hz.
"File systems" tab can update at 1 Hz.

It can't:

• be transparent and work as overlay
• monitor RSS over time for each process
• monitor GPU load and memory usage per process (install nvidia-smi for this or alternative)
• monitor GPU in detail like load, memory, current clocks, PCIe status (install nvidia-smi for this or alternative)
• display CPU affinity for each process
• set CPU affinity for each process

As user it can:

• list opened files system wide (with hamburger menu)
• list opened files per process (with context menu)
• display memory maps for each process
• STOP/CONTINUE/KILL processes with shortcuts

As super user it can:

• can change nice value for each process

Questionable default settings:

• smoothing on
• no process hierarchy
• no RSS column
• no status column
• no F5 hotkey for refresh
• quite bulky for OSD/overlay (560 × 341 pixels)