[tychver]

Nah, unfortunately simply moving the GC bits from the object itself to bitmap in the page header made Ruby CoW friendlier but not CoW friendly! Each Ruby page is 4x OS pages on Linux, so marking into the header of each Ruby page still causes 1/4 of the parent heap to copy into the child process.

The bigger issue is heap fragmentation. Lots of inherited pages have a couple of free slots that Ruby will happily consume, effectively causing you to pay a 4kb copy for a 40 byte object.

This means allocating a few hundred objects can cause basically the entire heap to copy. Combine this with Transparent Huge Pages, where flipping one bit causes a 2MB copy rather than 4kb, and a few hundred object allocations can cause the entire parent process memory to be duplicated into the child.

Aaron Patterson is doing some great work to bring GC compaction to MRI, which will help reduce fragmentation, but it's a huge task.