Ubuntu – Max number of simaltenous running process

12.04cpukernelprocess

The kernel says it can support up to 32768 process id at /proc/sys/kernel/pid_max, but how many process can my server handle simultaneously without complaining about resources or hangs my server,

I know it depends on each process behaviors and resources need, but is there some kind of equation that has some parameters like Ram, cache, cpu cores …etc?

Edit:

My Server is hosted on Linode with following specifications:

RAM: 12 GB
CPU: Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
cpu MHz     : 2499.970
cache size  : 4096 KB
Cores: 6 cores

My server has some old versions that run my application, 

Apache 2.2 
mysql 5.5
php 5.3
php5-fpm

Best Answer

There is a formula for calculating the maximum number of active PIDs or threads. Excerpt from kernel/fork.c:

/*
 * set_max_threads
 */
static void set_max_threads(unsigned int max_threads_suggested)
{

    u64 threads;

    /*
     * The number of threads shall be limited such that the thread
     * structures may only consume a small part of the available memory.
     */
    if (fls64(totalram_pages) + fls64(PAGE_SIZE) > 64)
            threads = MAX_THREADS;
    else
            threads = div64_u64((u64) totalram_pages * (u64) PAGE_SIZE,
                                (u64) THREAD_SIZE * 8UL);

    if (threads > max_threads_suggested)
            threads = max_threads_suggested;

    max_threads = clamp_t(u64, threads, MIN_THREADS, MAX_THREADS);
}

However, normally other limits will be hit first. If RAM and other resources last, then some cgroup limits will likely be first, where basically the limit is:

$ cat /sys/fs/cgroup/pids/user.slice/user-1000.slice/pids.max
12288

The number, 12288, is the same on both my older 3 gigabyte server and my newer 16 gigabyte server.
And I can test by trying to spin out more than the maximum number, resulting in a message in /var/log/kern.log:

Feb 12 15:49:11 s15 kernel: [  135.742278] cgroup: fork rejected by pids controller in /user.slice/user-1000.slice

And checking the number I had at the time:

$ cat /sys/fs/cgroup/pids/user.slice/user-1000.slice/pids.current
12287

top said about 12479

But after those processes ended, I got:

$ cat /sys/fs/cgroup/pids/user.slice/pids.current
15

top said about 205, and note: 12479 - 205 + 15 = 12289

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