Gwan - Why gwan determine only 1 core? - multicore

Question

I am currently testing the gwan web server, I have a question about the default settings for the gwan worker, and the CPU Core detection.
Running gwan on a Xen server (which contains a 4-core Xeon CPU), the gwan.log file reports that only got a single core was detected:
'./gwan -w 4' used to override 1 x 1-Core CPU(s)
[Wed May 22 06:54:29 2013 GMT] using 1 workers 0[01]0
[Wed May 22 06:54:29 2013 GMT] among 2 threads 0[11]1
[Wed May 22 06:54:29 2013 GMT] (can't use more threads than CPU Cores)
[Wed May 22 06:54:29 2013 GMT] CPU: 1x Intel(R) Xeon(R) CPU E5506 # 2.13GHz
[Wed May 22 06:54:29 2013 GMT] 0 id: 0 0
[Wed May 22 06:54:29 2013 GMT] 1 id: 1 1
[Wed May 22 06:54:29 2013 GMT] 2 id: 2 2
[Wed May 22 06:54:29 2013 GMT] 3 id: 3 3
[Wed May 22 06:54:29 2013 GMT] CPU(s):1, Core(s)/CPU:1, Thread(s)/Core:2
Any idea?
Thanks!!

Answer 1

the gwan.log file reports that I only got a single core:
Xen, like many other hypervisors, is breaking the CPUID instruction and the /proc/cpuinfo Linux kernel structures (both of which are used by G-WAN to detect CPU Cores).
As you have seen, this is a real problem for multithreaded applications designed to scale on multicore.
'./gwan -w 4' used to override 1 x 1-Core CPU(s)
To avoid stupid manual overrides wasting memory and impacting performance, G-WAN checks that the user-supplied thread count is not greater than the actual CPU Core count.
This is why you have the warning: "(can't use more threads than CPU Cores)".
To bypass this protection and warning, you can use the -g ("God") mode:
./gwan -g -w 4
This command line switch is documented in the G-WAN executable help:
./gwan -h
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Usage: gwan [-b -d -g -k -r -t -v -w] [argument]
(grouped options '-bd' are ignored, use '-b -d')
-b: use the TCP_DEFER_ACCEPT TCP option
(this is disabling the DoS shield!)
-d: daemon mode (uses an 'angel' process)
-d:group:user dumps 'root' privileges
-g: do not limit worker threads to Cores
(using more threads than physical Cores
will lower speed and waste resources)
-k: (gracefully) kill local gwan processes
using the pid files of the gwan folder
-r: run the specified C script (and exit)
for general-purpose code, not servlets
-t: store client requests in 'trace' file
(may be useful to debug/trace attacks)
-v: return version number and build date
-w: define the number of worker threads.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Available network interfaces (2):
127.0.0.1 192.168.0.11