FreeBSD SMP - kernel compilation bench

I made something like an application level benchmark to bench the performance of the FreeBSD SMP (Symmetric Multi Processing) kernel against the normal one.

My application was compiling my FreeBSD custom kernel. I tried several -j options for make to find out optimal values for that kind of equipment und to make best use out of FreeBSD-SMP.

I got interesting results by doing that under a normal FreeBSD-current kernel environment, a SMP kernel running only one CPU and doing that on a SMP kernel and using the two CPU's.

You can see this kind of tests already in PC magazines like C't, where they bench the compile time for a Linux kernel.

All test were performed in single user mode using FreeBSD-current's kernel sources of May, 1st 1997.

My test equipment

• Tyan Titan Pro ATX (S1668ATX)
• 2 x 200 MHz Pentium Pro CPU's
• 64 MB RAM, 60ns, PS/2 (2x32)
• AHA 2940, IBM DORS 32160

FreeBSD-current uni-processor kernel
jobs (make -j)	time real [s]	time user [s]	time sys [s]
1	262.09	189.47	15.21
2	215.58	191.09	16.24
4	213.17	191.39	19.49
8	213.24	191.91	19.40
16	215.19	192.10	19.31

FreeBSD-current custom SMP kernel 1 CPU
jobs (make -j)	time real [s]	time user [s]	time sys [s]
1	269.95	189.22	23.39
2	222.68	191.38	23.03
4	218.24	192.00	23.47
8	217.93	191.35	24.29
16	220.15	191.58	24.26

FreeBSD-current custom SMP kernel 2 CPUs
jobs (make -j)	time real [s]	time user [s]	time sys [s]
1	252.35	144.69	75.06
2	137.06	169.23	56.46
4	119.95	176.02	53.25
8	119.08	175.47	54.97
16	120.03	178.42	53.32

Some performance figures

Figure 1: real time in seconds

This figure shows two things:

• The FreeBSD SMP kernel with 2 CPU's is about 90% faster than a normal uni-processor system. Yeah ! That's factor 1.9 ;-)
• The benchmark results of the SMP kernel running with only one CPU are a little slower than running on a normal kernel.

Figure 2: user time in seconds

Although the SMP kernel with one CPU is a little slower, you see, that user time is about the same !

Figure 3: sys time (kernel mode) in seconds

• Here you see the little system overhead of the SMP kernel comparing normal kernel and one CPU SMP kernel.
• If both CPU's are running, the system time increases about factor 3-4. This must be the about 10% you are loosing if you compare the real times.
• But no matter, 90% faster is definitively faster ;-)