Why isn’t Ruby 1.8.7 copy-on-write friendly?

You might have heard, Ruby 1.8.7 isn’t copy-on-write (COW) friendly. Lets see why.

To set some context, say you have built an application in Rails using Ruby 1.8.7 and is deployed on a UNIX system. Scaling the application means spawning multiple processes to serve client requests. Since memory cannot be shared between processes, there is whole lot of memory wasted in holding the same Rails code in multiple processes. Modern UNIX systems provide fork functionality which let a child process share memory with the parent. So essentially there would be one parent process holding the, shared by all memory, and it would spawn child processes, which will share the parent’s memory. When something is being written to the shared memory by the parent, each child process is given a copy of that memory and shared memory in the parent process is written with new data. Similarly, when the child writes to the shared memory, the child gets its own copy of that memory and writes to it with the new data. This is called copy-on-write (COW) technique.

Now lets see how the Ruby 1.8.7 garbage collector works. It is apparently simple in its implementation. It uses a mark-and-sweep algorithm to collect unused memory. What this essentially means is, it scans each and every object and marks a flag “on” the object currently being used. At the end of this cycle, it sweeps or collects all the objects that have not been marked. But the problem here is that the “mark” information is stored on the object itself, in essence making each used object “dirty” which triggers a copy-on-write by the underlying UNIX system. All the used objects held in the shared memory, will be copied to each child’s memory space. This nullifies all the memory savings that would have been possible with the copy-on-write technique. This is why Ruby 1.8.7 isn’t copy-on-write friendly.

Ruby Enterprise Edition circumvents this problem by patching the GC to store the “mark” information outside the object. For more information checkout this.

For a longer and better explanation, go here.


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