From time to time, somebody critisizes OpenVZ kernel patch for its intrusiveness and size. Right, it is big and intrusive -- it adds a whole lot of new features into the kernel. But how big is it?

Our engineer prepared some stats on three different kernels:
1. OpenVZ stable kernel (based on 2.6.18-RHEL5);
2. OpenVZ development kernel (based on 2.6.27);
3. RHEL5.3 kernel (based on 2.6.18).
You can see the results by clicking the image at the right.

Some notes for the graph. For OpenVZ kernels, we distinguish between core kernel changes and the stuff that is built as modules. For RHEL kernel, we break the patchset down into a few categories, such as drivers, Xen, GFS, ext4 and so on; "other" means everything not covered by any other category. The numbers are thousands lines of code added and deleted, combined. A table below the graph has some more details, like how many files were changed, how many lines added and deleted.

Now to the conclusions. Two major points can be made:
1. Even without drivers, RHEL5 kernel patches add/delete 434 KLOCs*, which is 8.5x times bigger then OpenVZ kernel modifications (51 KLOC). So, yes, OpenVZ patch set is big, but not that big.
2. OpenVZ based on mainstream 2.6.27 kernel requires 40% less** modifications to the kernel due to on-going effort to integrate the functionality into mainstream.

* KLOC is a thousand lines of source code.
** we only count the core changes, omitting the modules.