udevd: change the default value of udev.children-max (again)

Follow-up for faae64fa3d, which increased the
default number of udev workers per cpu regardless of how big the system is.

It's not really clear from the commit message if the new number of workers
improved the overall time for the boot process or only reduced the number of
times the max number of children limit was reached (and in this case
5406c36844 commit might have been more appropriate in the first place).

But systems with ~1000 CPUs are not rare these days and the worker numbers get
quite large with CPU factor of 8. Spawning more than 2000 workers can't be
healthy on any system, no matter how big.

Indeed the main mistake is the belief that udev is CPU-intensive, and thus the
number of allowed workers has to increase with the number of CPUs. It is not,
at probably has never been. It's I/O bound, and sometimes, bound by resources
such as locks.

This is an argument to:

 - scale only weakly with the number of CPUs, and the rationale to switch back
   to a scale factor C=2 but with a higher offset number which should affect
   systems with a small number of CPUs only. With this patch applied the offset
   is increased from O=8 to O=16.

 - put an absolute maximum limit to make sure no more than 2048 workers are
   spawned no matter how big the system is.

This still provides more workers for the laptop cases (where the number of CPUs
is limited), while avoiding sky-rocketing numbers for big systems.

Note that on most desktop systems, the memory limit will kick in. The following
table collects numbers about children-max. For each scenario, the first column
is the "cpu_limit" limit, and the second number is the minimum amount of memory
for the "cpu_limit" limit to become relevant (with less RAM, memory will limit
the number of children thus "mem_limit" will become the active limit).

       |    > v240    |    < v240     |  this patch   |
 CPUs  | C = 8, O = 8 | C = 2, O = 8  | C = 2, O = 16 |
-------------------------------------------------------
   1   |   16      2  |   10    1.3   |   18       2  |
   2   |   24      3  |   12    1.5   |   20       2  |
   4   |   40      5  |   16      2   |   24       3  |
   8   |   72      9  |   24      3   |   32       4  |
  16   |  136     17  |   40      5   |   48       5  |
  64   |  520     65  |  136     17   |  144      18  |
1024   | 8200   1025  | 2056    263   | 2048     256  |
2048   |16392   2049  | 4104    513   | 2048     256  |

This patch is mainly based on Martin Wilck's analyze and comments.

src/udev/udevd.c

@@ -69,6 +69,8 @@
 #include "udev.h"
 #include "user-util.h"
 
+#define WORKER_NUM_MAX 2048U
+
 static bool arg_debug = false;
 static int arg_daemonize = false;
 static ResolveNameTiming arg_resolve_name_timing = RESOLVE_NAME_EARLY;
@@ -1745,16 +1747,18 @@ static int run(int argc, char *argv[]) {
                 return r;
 
         if (arg_children_max == 0) {
+                unsigned long cpu_limit, mem_limit;
+                unsigned long cpu_count = 1;
                 cpu_set_t cpu_set;
-                unsigned long mem_limit;
-
-                arg_children_max = 8;
 
                 if (sched_getaffinity(0, sizeof(cpu_set), &cpu_set) == 0)
-                        arg_children_max += CPU_COUNT(&cpu_set) * 8;
+                        cpu_count = CPU_COUNT(&cpu_set);
 
-                mem_limit = physical_memory() / (128LU*1024*1024);
-                arg_children_max = MAX(10U, MIN(arg_children_max, mem_limit));
+                cpu_limit = cpu_count * 2 + 16;
+                mem_limit = MAX(physical_memory() / (128UL*1024*1024), 10U);
+
+                arg_children_max = MIN(cpu_limit, mem_limit);
+                arg_children_max = MIN(WORKER_NUM_MAX, arg_children_max);
 
                 log_debug("Set children_max to %u", arg_children_max);
         }