probably unrelated to SMP, I've tested it successfully on a dual Opteron and a dual Xeon system.

Best Regards

I run it on plenty of SMP systems as well.  We'll need some more detailed info to do anything here.

If you do not mind, please try our litespeed web server at http://www.litespeedtech.com/, the pre-built APC binary works fine on uni-processor system, but failed on SMP system.

Redhat 8 with kernel 2.4.22smp kernel is used on our test machine. And we had test it on a dual opteron with 2.6 kernel as well as on a dual Sun Ultra 60 with Solaris 8. 
All are failed. 

I am not sure what we did wrong as we followed the instruction to built apc.so

Thanks,
George Wang

Feature it out finally, it is because the maximum memory size of PHP process is limit to 30M and APC want to allocate 30M shared memory by default. :-)

APCs shared memory usage shouldn't go against php's 
memory limit.  Most likely your kernel setting shmmax is 
under configured.  Do

sysctl kernel.shmmax

Likely it's below what you're asking APC to allocate 
(30M by default)/

shared memory usage probably does not counted for process memory usage on an uniprocessor system, but it does on a SMP system, because APC works with the same process memory limit on an uniprocessor system. 
After we rise the process memory limit, APC successfully allocate 30M shared memory.

result of command "sysctl kernel.shmmax" on our SMP linux box is 32M. :-)