From Rob Hoelz:

I did some additional testing of ack using multiple workers - starting with a ondemand forking model, and then moving to a pre-fork model. The former didn’t provide too much of a benefit, and the latter was somewhat complicated. At first I thought maybe I could have a shared work queue pipe, but I realized that each child process would probably buffer inputs, so the work would get divied up oddly. I then had the thought to maintain a list of available children for work - children would send a SIGUSR1 to their parent when done, and the parent would re-register the corresponding child in the availability list. A good idea in theory, but it turns out that even though si_pid is documented as occurring in siginfo_t, it either isn’t available on Linux, or the Perl POSIX module doesn’t pass it through. I ended up opting for a simple list of children and iterating through them in a round-robin fashion. A possible improvement could be random selection, perhaps weighted by the sizes of the files that each child has handled thus far.

The multi-process model didn’t show large benefits - although my first experiment didn’t actually perform any work. When I added experiments to actually test against regexes, that’s when I saw about a 200% speedup running with 8 jobs (8 being the number of cores in my machine). I tried 16 (figuring one job could be waiting on I/O while the other used the CPU), but actually saw a slowdown as a result. This might be different on HDDs, though.

Another big improvement was to use sysread with a 128K buffer (which is the default cat uses) rather than Perl’s diamond operator - this combined with the multi-process stuff lead to a 400% speedup.