Some software changes it's behavior based on capabilities of the system it's running on.
but sometimes it's interesting to check how a software would heave on a different system, which is not at hand right now.
On Linux, a lot of information about the current system can be found in
/proc and
/sys.
These filesystems are virtual, so they can not changed easily with an editor.
In my case I want to simulate a lot more CPUs.
These are visible in several locations.
The most know is probably
/proc/cpuinfo. There you find a block of information for each CPU the kernel knows about. Based on the current configuration, I create a modified fake file somewhere in a different space:
#!/bin/bash
# cpus.sh
count=0
max_socket=8
max_core=32
END=5
for ((soc=0;soc<max_socket;soc++)); do
for (( cor=0;cor<max_core;cor++)); do
echo "processor : $count
vendor_id : GenuineIntel
cpu family : 6
model : 37
model name : Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz
stepping : 1
microcode : 0x3b
cpu MHz : 2596.103
cache size : 25600 KB
physical id : $soc
siblings : $max_core
core id : $cor
cpu cores : $max_core
apicid : 0
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 11
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts mmx fxsr sse sse2 ss syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts nopl xtopology tsc_reliable nonstop_tsc aperfmperf pni pclmulqdq ssse3 cx16 sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes hypervisor lahf_lm ida arat epb pln pts dtherm pti tsc_adjust
bugs : cpu_meltdown spectre_v2
bogomips : 5193.98
clflush size : 64
cache_alignment : 64
address sizes : 42 bits physical, 48 bits virtual
power management:
"
let count=count+1
done
done
and create a file with
./cpus.sh>cpuinfo.256
There is another location as well:
/sys/devices/system/cpu.
In this directory are several directories and files with interesting information.
I copy the fill directory to another place (ignoring all the errors).
First the number of cpu[id] directories might need adjustment.
In my case a simple set of symlinks is sufficient:
for i in {2..255} ; do
echo $i
ln -s cpu1 cpu${i}
done
In every cpu[id] durectory there is a symlinik to which node it belongs:
node0 -> ../../node/node0
So it might be required to spoof proper entries in
/sys/devices/system/node. In my case it's not required.
The last fix required in my case is in the file
cpu/online.
It contains
0-255 now (instead of
0-2).
As I mentioned above the original files can not be manipulated as they are not real files.
The mount option
--bind does the trick:
mount --bind <my_working_dir>/cpuinfo.256 /proc/cpuinfo
mount --bind <my_working_dir>/cpu /sys/devices/system/cpu
After these nice manipulations, my sandbox Oracle instance shows now plenty of CPUs:
SQL> show parameter cpu_count
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
cpu_count integer 256
Update (2018-03-21):
For Oracle Databases I got 2 hints how to make it calculate with more CPUs than really available.
with this small stap script:
#!/usr/bin/stap
function modify_rax() %{ long ret; ret = 6; memcpy( ((char *)CONTEXT->uregs) + 80 , &ret, sizeof(ret)); %}
probe process(“oracle”).function(“skgpnumcpu”).return { modify_rax(); }
and
