【转】How Google uses Linux?

KS2009: How Google uses Linux
By Jonathan Corbet
October 21, 2009
LWN's 2009 Kernel Summit coverage
There may be no single organization which runs more Linux systems than Google. But the kernel development community knows little about how Google uses Linux and what sort of problems are encountered there. Google's Mike Waychison traveled to Tokyo to help shed some light on this situation; the result was an interesting view on what it takes to run Linux in this extremely demanding setting.
Mike started the talk by giving the developers a good laugh: it seems that Google manages its kernel code with Perforce. He apologized for that. There is a single tree that all developers commit to. About every 17 months, Google rebases its work to a current mainline release; what follows is a long struggle to make everything work again. Once that's done, internal "feature" releases happen about every six months.
This way of doing things is far from ideal; it means that Google lags far behind the mainline and has a hard time talking with the kernel development community about its problems.
There are about 30 engineers working on Google's kernel. Currently they tend to check their changes into the tree, then forget about them for the next 18 months. This leads to some real maintenance issues; developers often have little idea of what's actually in Google's tree until it breaks.
And there's a lot in that tree. Google started with the 2.4.18 kernel - but they patched over 2000 files, inserting 492,000 lines of code. Among other things, they backported 64-bit support into that kernel. Eventually they moved to 2.6.11, primarily because they needed SATA support. A 2.6.18-based kernel followed, and they are now working on preparing a 2.6.26-based kernel for deployment in the near future. They are currently carrying 1208 patches to 2.6.26, inserting almost 300,000 lines of code. Roughly 25% of those patches, Mike estimates, are backports of newer features.
There are plans