I copied a 16 GB USB memory stick to another 16 GB stick (cloned it), by using
the following command:

It took about 27 minutes, and the cloning works. But during the entire copy
process, the mouse and cursor movement become erratic, and lag terribly. This
makes using the laptop during the copy almost impossible.

This happened to me on a previous occasion, so the effect is reproducible.

What causes this effect?
How do I stop the erratic behaviour?
Are you able to replicate the behaviour on your computer?

Limited resource condition I assume.

See the man pages for nice, renice and ionice.

https://www.zylk.net/en/web-2-0/blog...-safer-than-cp

It boils down to laptop, usb and the copy program.

I assume it's because the mouse and the USB drive are on the same USB bus, and the USB drive is saturating it.

The kernel is swapping out programs to use memory for disk caching. You can lower the value of /proc/sys/vm/swappiness to as little as 1 to preclude this.

Ed

I would second this suggestion. This issue actually came up in our virtual LUG meeting a couple of hours ago.

As noted above, you can use "nice cp ..." to run this command. This voluntarily assigns the process a reduced priority, so that it will let other processes which are competing against it (such as your UI ...) "play through." (Which is, after all, a "nice, courteous" thing for resource-intensive processes to do ...)

Interestingly, my experience is that this really doesn't affect the completion time of most things very much. There's usually an abundance of CPU and I/O resources to spare. So, this command simply causes the "nice" process to step aside when challenged. When you move your mouse and the UI needs to redraw the mouse-pointer quickly, the "nice" activity doesn't get in the way.

That's where ionice comes in idoes the same thing as nice/renice but for io operations

Heartily agree with that. (I had overlooked "ionice.")

It certainly wouldn't hurt to say nice && ionice && cp ...

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That assumes that swap space was even allocated. If I got a nickel for every time I ran across a post on the 'Net effectively saying "Nah... you don't need swap".

Swapping is the most likely explanation. The kernel's default swappiness (60) is too high for desktop use. Desktop users need to lower the swappiness to prevent the kernel from swapping out programs during large file copies.

In contrast, lowering the priority is not likely to help because the OP's copy is transferring only 10MB/s (the speed of the USB flash). This rate is far too slow to saturate USB2 or anything else in the computer.
Ed