We have some code that relies on extensive usage of fork. We started to hit the performance problems and one of our hypothesis is that we do have a lot of speed wasted when copy-on-write happens in the forked processes.
Is there a way to specifically detect when and how copy-and-write happens, to have a detailed insight into this process.
My platform is OSX but more general information is also appreciated.
There are a few ways to get this info on OS X. If you're satisfied with just watching information about copy-on-write behavior from the command-line, you can use the vm_stat tool with an interval. E.g., vm_stat 0.5 will print full statistics twice per second. One of the columns is the number of copy-on-write faults.
If you'd like to gather specific information in a more detailed way, but still from outside the actual running process, you can use the Instruments application that comes with OS X. This includes a set of tools for gathering information about a running process, the most useful of which for your case are likely to be the VM Tracker, Virtual Memory Trace, or Shared Memory instruments. These capture lots of useful information over the lifetime of a process. The application is not super intuitive, but it will do what you need.
If you'd like detailed information in-process, I think you'll need to use the (poorly documented) VM statistics API. You can request that the kernel fill a vm_statistics struct using the host_statistics routine. For example, running this code:
mach_msg_type_number_t count = HOST_VM_INFO_COUNT;
vm_statistics_data_t vmstats;
kern_return_t host_statistics(mach_host_self(), HOST_VM_INFO, (host_info_t) &vmstats, &count);
will fill the vmstats structure with information such as cow_faults, which gives the number of faults triggered by copy-on-write behavior. Check out the headers /usr/include/mach/vm_*, which declare the types and routines for gathering this information.