Reddit was initially built on CMUCL on FreeBSD. Here is what Steve Huffman said on why they switched to python one weekend.
"The biggest trouble that plagued us was that we could never quite get Lisp reddit stable enough to sleep at night. There were weird threading issues that would bring the site to its knees a couple times a day and required constant monitoring."
I used CMUCL a bit, and SBCL almost never. Does SBCL really deliver on what it set out to do? Is it better, faster, more portable? As far as I know, the Windows port never matured.
Which of the two do people recommend using?
SBCL set out to make a version of CMUCL that was easier to build and maintain. I'm pretty sure that was delivered.
But of course that's not of much interest to the average user. I think SBCL is in general faster, is developed more actively, has more features that are relevant in the modern world (eg. lose a Motif interface, gain native threads), and currently runs on more platforms than CMUCL does.
My impression is that most people using CMUCL are ones who started using it 10 years ago and are comfortable with it, new users would tend to go with SBCL. But I'm biased, so it's quite possible that there are real and substantial reasons to prefer CMUCL.
I was subscribed to SBCL list some months ago. I could see that Windows port was quickly progressing. "Never matured" suggest a project that was tried and later abandoned. It seems more of an ongoing effort. Versions are still being created regularly. I would try in a few months.
"The biggest trouble that plagued us was that we could never quite get Lisp reddit stable enough to sleep at night. There were weird threading issues that would bring the site to its knees a couple times a day and required constant monitoring."
You can read more of it in this blog entry.
http://www.findinglisp.com/blog/2005/12/reddit-and-lisp-psyc...
Edit: Corrected :-)