[BENCHMARK] nproc: Look Ma, No get_tgid_list!

[Roger Luethi]

This posting demonstrates a new method of monitoring all processes in
a large system.

You may remember what a /proc based tool does when monitoring some
10^5 processes -- it spends its time in the kernel hanging on to a
read task_list_lock:

==> 10000 processes: top -d 0 -b > /dev/null <==
CPU: CPU with timer interrupt, speed 0 MHz (estimated)
Profiling through timer interrupt
samples  %        image name               symbol name
35855    36.0707  vmlinux                  get_tgid_list
9366      9.4223  vmlinux                  pid_alive
7077      7.1196  libc-2.3.3.so            _IO_vfscanf_internal
5386      5.4184  vmlinux                  number
3664      3.6860  vmlinux                  proc_pid_stat
3077      3.0955  libc-2.3.3.so            _IO_vfprintf_internal
2136      2.1489  vmlinux                  __d_lookup
1720      1.7303  vmlinux                  vsnprintf
1451      1.4597  libc-2.3.3.so            __i686.get_pc_thunk.bx
1409      1.4175  libc-2.3.3.so            _IO_default_xsputn_internal
1258      1.2656  libc-2.3.3.so            _IO_putc_internal
1225      1.2324  vmlinux                  link_path_walk
1210      1.2173  libc-2.3.3.so            ____strtoul_l_internal
1199      1.2062  vmlinux                  task_statm
1157      1.1640  libc-2.3.3.so            ____strtol_l_internal
794       0.7988  libc-2.3.3.so            _IO_sputbackc_internal
776       0.7807  libncurses.so.5.4        _nc_outch

Here's a profile for an nproc based tool monitoring the same set
of processes:

==> 10000 processes: nprocbench <==
CPU: CPU with timer interrupt, speed 0 MHz (estimated)
Profiling through timer interrupt
samples  %        app name                 symbol name
8641     24.8626  vmlinux                  __task_mem
2778      7.9931  vmlinux                  find_pid
2536      7.2968  vmlinux                  finish_task_switch
1872      5.3863  vmlinux                  netlink_recvmsg
1637      4.7101  vmlinux                  nproc_pid_fields
1373      3.9505  vmlinux                  __wake_up
1218      3.5045  vmlinux                  __copy_to_user_ll
1134      3.2628  vmlinux                  __task_mem_cheap
944       2.7162  vmlinux                  mmgrab
876       2.5205  vmlinux                  nproc_ps_do_pid
568       1.6343  vmlinux                  skb_dequeue
526       1.5135  libc-2.3.3.so            __recv
514       1.4789  vmlinux                  alloc_skb
510       1.4674  vmlinux                  __might_sleep
485       1.3955  vmlinux                  skb_release_data
463       1.3322  vmlinux                  netlink_attachskb
363       1.0445  vmlinux                  sys_recvfrom

Resource usage is now dominated by field computation, rather than by
delivery overhead. By now it should be clear that nproc is not only a
cleaner interface with lower overhead for tools, it also scales a lot
better than /proc.

Roger
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