What are the factor on which PID_MAX depends in Linux

Question

I have checked two linux systems, both were of 64 bit but PID_MAX is different for both of them.

On one system(SYSTEM1)

cat /proc/sys/kernel/pid_max
32768

SYSTEM1 configuration:

dmesg | grep -i smp.*allow

getconf PAGESIZE
4096

cat /proc/cpuinfo | awk '/^processor/{print $3}' | tail -1
11
cat /proc/cpuinfo
processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 2201.000
cache size  : 15360 KB
physical id : 0
siblings    : 6
core id     : 0
cpu cores   : 6
apicid      : 0
initial apicid  : 0
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4389.48
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 1
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 2201.000
cache size  : 15360 KB
physical id : 0
siblings    : 6
core id     : 1
cpu cores   : 6
apicid      : 2
initial apicid  : 2
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4389.48
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 2
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 2201.000
cache size  : 15360 KB
physical id : 0
siblings    : 6
core id     : 2
cpu cores   : 6
apicid      : 4
initial apicid  : 4
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4389.48
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 3
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 1300.000
cache size  : 15360 KB
physical id : 0
siblings    : 6
core id     : 3
cpu cores   : 6
apicid      : 6
initial apicid  : 6
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4389.48
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 4
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 2201.000
cache size  : 15360 KB
physical id : 0
siblings    : 6
core id     : 4
cpu cores   : 6
apicid      : 8
initial apicid  : 8
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4389.48
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 5
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 1200.000
cache size  : 15360 KB
physical id : 0
siblings    : 6
core id     : 5
cpu cores   : 6
apicid      : 10
initial apicid  : 10
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4389.48
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 6
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 2201.000
cache size  : 15360 KB
physical id : 1
siblings    : 6
core id     : 0
cpu cores   : 6
apicid      : 32
initial apicid  : 32
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4392.34
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 7
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 2201.000
cache size  : 15360 KB
physical id : 1
siblings    : 6
core id     : 1
cpu cores   : 6
apicid      : 34
initial apicid  : 34
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4392.34
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 8
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 2201.000
cache size  : 15360 KB
physical id : 1
siblings    : 6
core id     : 2
cpu cores   : 6
apicid      : 36
initial apicid  : 36
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4392.34
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 9
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 2201.000
cache size  : 15360 KB
physical id : 1
siblings    : 6
core id     : 3
cpu cores   : 6
apicid      : 38
initial apicid  : 38
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4392.34
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 10
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 1200.000
cache size  : 15360 KB
physical id : 1
siblings    : 6
core id     : 4
cpu cores   : 6
apicid      : 40
initial apicid  : 40
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4392.34
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

processor   : 11
vendor_id   : GenuineIntel
cpu family  : 6
model       : 45
model name  : Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
stepping    : 7
microcode   : 1803
cpu MHz     : 1200.000
cache size  : 15360 KB
physical id : 1
siblings    : 6
core id     : 5
cpu cores   : 6
apicid      : 42
initial apicid  : 42
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4392.34
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

Whereas on other(SYSTEM2), it was

 cat /proc/sys/kernel/pid_max
65536

SYSTEM2 configuration:

dmesg | grep -i smp.*allow

SMP: Allowing 64 CPUs, 61 hotplug CPUs

getconf PAGESIZE
4096

cat /proc/cpuinfo | awk '/^processor/{print $3}' | tail -1
2

cat /proc/cpuinfo
processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 23
model name  : Intel(R) Xeon(R) CPU           X5450  @ 3.00GHz
stepping    : 6
microcode   : 4294967295
cpu MHz     : 2992.467
cache size  : 6144 KB
physical id : 0
siblings    : 3
core id     : 0
cpu cores   : 3
apicid      : 0
initial apicid  : 0
fpu     : yes
fpu_exception   : yes
cpuid level : 10
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx lm constant_tsc rep_good unfair_spinlock pni ssse3 cx16 sse4_1 hypervisor lahf_lm
bogomips    : 5984.93
clflush size    : 64
cache_alignment : 64
address sizes   : 38 bits physical, 48 bits virtual
power management:

processor   : 1
vendor_id   : GenuineIntel
cpu family  : 6
model       : 23
model name  : Intel(R) Xeon(R) CPU           X5450  @ 3.00GHz
stepping    : 6
microcode   : 4294967295
cpu MHz     : 2992.467
cache size  : 6144 KB
physical id : 0
siblings    : 3
core id     : 1
cpu cores   : 3
apicid      : 1
initial apicid  : 1
fpu     : yes
fpu_exception   : yes
cpuid level : 10
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx lm constant_tsc rep_good unfair_spinlock pni ssse3 cx16 sse4_1 hypervisor lahf_lm
bogomips    : 5984.93
clflush size    : 64
cache_alignment : 64
address sizes   : 38 bits physical, 48 bits virtual
power management:

processor   : 2
vendor_id   : GenuineIntel
cpu family  : 6
model       : 23
model name  : Intel(R) Xeon(R) CPU           X5450  @ 3.00GHz
stepping    : 6
microcode   : 4294967295
cpu MHz     : 2992.467
cache size  : 6144 KB
physical id : 0
siblings    : 3
core id     : 2
cpu cores   : 3
apicid      : 2
initial apicid  : 2
fpu     : yes
fpu_exception   : yes
cpuid level : 10
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx lm constant_tsc rep_good unfair_spinlock pni ssse3 cx16 sse4_1 hypervisor lahf_lm
bogomips    : 5984.93
clflush size    : 64
cache_alignment : 64
address sizes   : 38 bits physical, 48 bits virtual
power management:

From man pages.

proc/sys/kernel/pid_max This file (new in Linux 2.5) specifies the value at which PIDs wrap around (i.e., the value in this file is one greater than the maximum PID). The default value for this file, 32768, results in the same range of PIDs as on earlier kernels. On 32-bit platfroms, 32768 is the maximum value for pid_max. On 64-bit systems, pid_max can be set to any value up to 2^22 (PID_MAX_LIMIT, approximately 4 million).

Please note, both of these system were having same linux kernel but SYSTEM2 was running on hyper-V.

Now, my question is what are factors which are responsible for changing pid_max? Thank you in advance.

Answer 1

pid_max is set based on the number of possible CPUs, which you can see in the kernel source in pid.c line 595

pid_max = min(pid_max_max, max_t(int, pid_max,
           PIDS_PER_CPU_DEFAULT * num_possible_cpus()));

num_possible_cpus() returns the maximum number of CPUs that could be in the system (including hotplug CPUs that could be plugged in after boot).

As you can see the expression includes a minimum and maximum.

The minimum is the initial value of pid_max, which is derived from the macro PID_MAX_DEFAULT, which will be either 0x1000 or 0x8000 depending on the compile-time configuration setting CONFIG_BASE_SMALL.

The maximum is pid_max_max, which in turn is derived from some kernel compile-time configuration settings, specifically CONFIG_BASE_SMALL and PAGE_SIZE. This is defined in the macro PID_MAX_LIMIT see threads.h line 33:

#define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
    (sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT))

So putting all this together in an example: on my system here (Centos 6):

• PAGESIZE is 4096
• CONFIG_BASE_SMALL is 0
• PID_MAX_DEFAULT would therefore be 0x8000 (32768)
• PID_MAX_LIMIT would be 4 * 1024 * 1024 = 4194304
• max_pid would therefore be in the range 4194304 >= max_pid >= 32768

From userland, you can inspect the number of possible CPUs in file /sys/devices/system/cpu/possible. The format of the data in this file is a little strange - it will be a string of the form 0-max, for example 0-15 would mean a maximum of 16 CPUs (noting that the number is zero counting).

For more information about this file and the other files in this directory, see cputopology.txt.

For more information about CPU hotplugging in general, see cpu-hotplug.txt

If you want to check the kernel compile time configuration settings, check this answer for some tips.

NB. Kernel line numbers given are valid for Kernel 4.7.10.