CODEWITHSUNDEEP
ubuntuparallel-processing
Eugen
Eugen

Reputation: 2370

How many parallel user processes can there be?

Does anyone know how to estimate the remaining parallel threads for user processes on a machine with N cores with hyper-threading enabled (i.e. there are 2*N parallel jobs). How many cores are taken up by the OS, e.g. Ubuntu?

Upvotes: 0

Views: 207

Answers (1)

Dima Chubarov
Dima Chubarov

Reputation: 17169

Your OS implements preemptive multitasking. This means in particular, that no thread is getting hold of a core exclusively. Instead the slices of CPU time are distributed between the threads that are ready to continue. Such redistribution occurs many times within each second.

The uptime command in Linux can tell you how many threads on average were ready to run within the last 1, 5 and 15 minute intervals.

    $ uptime
    23:18pm  up  15:17,  3 users,  load average: 0.73, 0.62, 0.74

Here we have about 0.7, less than 1 thread on average. Not a very busy machine.

On a busy machine this number may exceed the number of available logical CPUs. Therefore if your 2n user processes are indeed always ready to use the CPU and do not give it away because they are waiting for data from RAM or I/O, then the number of operating system threads can be estimated as 

   current load average - 2n

In the example above this formula would not work, since user processes are waiting for user input or I/O and the CPU is mostly idle.

Upvotes: 1

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