I have a bug in my Linux app that is reproducable only on single-core CPUs. To debug it, I want to start the process from the command line so that it is limited to 1 CPU even on my multi-processor machine.
Is it possible to change this for a particular process, e.g. to run it so that it does not run (its) multiple threads on multiple processors?
You can use taskset from util-linux.
The masks may be specified in hexadecimal (with or without a leading "0x"), or as a CPU list with the --cpu-list option. For example,
0x00000001 is processor #0, 0x00000003 is processors #0 and #1, 0xFFFFFFFF is processors #0 through #31, 32 is processors #1, #4, and #5, --cpu-list 0-2,6 is processors #0, #1, #2, and #6. When taskset returns, it is guaranteed that the given program has been scheduled to a legal CPU.
As mentioned in the answer by @Ipor to limit program to one CPU hardware core/thread one may use taskset 1 prog [args]
Also, there is a way to explicitly prohibit a process from cloning (not sure about apps that use fork though, checked only for clone sys call).
The program to limit number of processes a program can have (threads) is prlimit --nproc=1 prog [args]. I've tried to use it with rsync and got "fork unavailable... error in IPC" and the end - rsync is written not to work as one thread.
strace prlimit --nproc=1 rsync
Running strace has shown as described in SO link below return value from clone call is -1 EAGAIN (resource temporary unavailable).
P.S. idea taken from https://stackoverflow.com/questions/38637451/is-there-a-way-force-a-program-to-use-only-1-thread.
std::thread::hardware_concurrency,sysconf(_SC_NPROCESSORS_ONLN),/proc/cpuinfo,cpuid, etc) there may be different approaches to fake the core count. That said, as suggested by @JohanMyréen, at that point it may be easier to hardcode how many threads to spawn.