FreeBSD or Linux for your MySQL Server?

This is an issue that comes up all the time at work. It is an issue for roughly four reasons:
Yahoo is a FreeBSD shop
Someone has heard that MySQL runs better on Linux
Someone knows that we run some of our servers on Linux rather than FreeBSD, and Yahoo is a FreeBSD shop (see #1), so they start to infer #2.
Most of the MySQL development is done on Linux and Windows. The only "FreeBSD guy" at MySQL AB seems to have left sometime in the last year or so.
This is an attempt to answer that question for folks at work and elsewhere.
The Short Version
I'll start with the simple answer.
If you don't have a preference, use Linux. You'll be happier. Trust me.
If, on the other hand, you'd like to stick with FreeBSD for some reason (and there are good reasons--one is noted later), read on.
The Problem: Threading
Having said that, let's look at the issues a little more closely. FreeBSD is a great operating system, but it has one important weakness that MySQL is very good at highlighting--threading support. FreeBSD's threads implementation isn't very good. I won't say that it sucks, because it could be a lot worse.
How Linux Threads
Threads on Linux are created using the clone() call, which is similar to fork(). You end up with a separate process in the process table, but the memory is shared among the processes. This means that the kernel gets involved in scheduling. From the kernel's point of view, they're all just processes. Many folks refer to this as kernel threading even though it's different than what the Solaris kernel does (for example). Some call that real kernel threading. :-)
Anyway, the LinuxThreads FAQ goes into a bit more detail. LinuxThreads is a library, available for other platforms--including FreeBSD. We'll come back to that in a bit.
How FreeBSD Threads
FreeBSD implements user-level threads. That means the kernel isn't aware of the threads and doesn't get involved in scheduling. Instead all the work is done in user space rather than kernel space. When your run top or ps on a machine that does this, unlike like in Linux, you'll see a single process rather than one per thread.
This is discussed a bit here in relation to LinuxThreads, which we'll get to.
Note that in FreeBSD 5.x, this may all be fixed. Time will tell. There is lot of working going on in the area of threading and kernel scheduling for FreeBSD 5.x.
FreeBSD's Threading Problems
Having run MySQL on various flavors of FreeBSD for the last 2.5 years, I can say that it has been a bumpy ride at times. Versions older than 4.2 (or maybe 4.3) have serious problems. I had a test case that could kill a MySQL server running on older versions of FreeBSD in a matter of minutes.
On more modern FreeBSD, things are better but not perfect. All the problems we've encountered at Yahoo seem to fall into 4 buckets.
1. Non-thread safe DNS Lookups
Certain operations are not thread-safe on FreeBSD. A fine example of that is gethostbyname(), which MySQL calls to convert host names in to IP addresses. Usually this happens for each new connection to the server and whenever MySQL needs to contact another machine--typically a replication slave connecting to its master.

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