I have a process that runs at about 100% CPU. So I run it with nice:
$ nice /dir/myamd.sh
The top command shows that my process still runs close to 100% (maybe slightly low like 99%). Is the nice command really working?
Thank you all for your responses!
Can I use both nice and cpulimit? Because I want my process yield to others AND take only 80% CPU for example.
Nice alters the relative priority of the process. If there is nothing else that needs the CPU then there is still nothing else with a higher priority, so it can continue to use 100% of the CPU.
Lots of modern machines have multiple cores, so if you have 4 cores but only 3 processes that are able to run then this is another example where the process could use 100% of a core regardless of the nice level.
top and ps lax will display the current nice level of all the processes on the system in the NI column.
Think of the CPU as a cashier at a checkout counter at a supermarket. If you have a multi-core CPU, there are several of these.
Each counter has a queue of customers waiting to pay for their groceries. This is the run queue of the CPU.
The length of the queue is the current CPU load. That's the load on the person behind the counter at this moment.
A person in the queue may be nice to the people behind, being totally fine with letting them get past them in the queue if they really need to hurry.
The cashier at the counter still has the same amount to work to do, and still works at the same pace (100%, until the queue is empty), but the people in the queue will arrive at the counter in an order depending on, amongst other things, how nice they are to other people in the queue.
In your case, the supermarket may be quite empty, but there's this one single customer with his really big trolley that needs to check out, and the cashier is working as hard as they can to process all the stuff he's buying. He is very nice though... but that doesn't really matter, the cashier won't slow down to have time for a chat.
On a Unix system, as opposed to at a supermarket, the CPU will only handle a process for a slice of time before sending them back into the queue. This is so that one single process won't block others.
You may even squeeze in context switching into this analogy. This is when the CPU switches from handling one process to handling another, and it can be time consuming to do it too often. This is basically what happens in-between serving customers at the counter; the time taken for the processed groceries to be put into bags etc. and the time taken for the next customer to move forwards, say "hi" etc.
Note: I'm using "CPU" above where in some cases I should say "kernel". From a user experience point-of-view, there's no difference.
The nice command is working as designed, just not as you expect it to.
Should you really want to cap the CPU usage and potentially waste available CPU resources, you might use:
cpulimit -l 25 /dir/myamd.sh
cpulimit will monitor myadm.sh CPU usage and throttle it using SIGSTOP/SIGCONT to keep its average usage under the 25% (of one vCPU) limit. On multi-core/threads systems, you can limit the usage to more than 100%.
A more efficient alternative is to use cgroups, see for example this blog for details.
You can use the ps command to check that you have correctly niced down your command:
$ nice -n 15 sm WILLY\ WILLIAM\ -\ Ego.m4a &> /dev/null &
[1] 29426
$ ps -lp $!
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
0 S 1000 29426 2279 1 95 15 - 141822 poll_s pts/7 00:00:00 sm
You can see the niceness level (15, in this case) in the aptly named NI column. Once this is done, you should relax: all that is happening is that the program is running without competition for the use of system resources, if it still attains 100% of CPU.
In your example, it is probably not working.
You're not running this program in the background (&). So this is probably the only program running on this TTY (apart from the shell that started it).
If you run a second program on a different TTY - e.g. a different terminal window - the relative nice levels of the two programs will have no visible effect.
niceis not meant to control the processing speed but the priority. If nothing else is running, then it remains the highest priority running task.