Re: Fine-grained locking for POSIX local sockets (UNIX domain sockets)


On Sun, 7 May 2006, Mike Jakubik wrote:

The difference in performance is just ridiculous. Is mysql written to be slow on freebsd or is there a problem with freebsd?

In past discussion, I think a reasonable conclusion has been some amount of both. We've identified a few particular areas where there are differences deserving attention:

- MySQL has increasingly made use of time query calls, especially in more
recent versions. On FreeBSD, we provide very precise time at somewhat
higher cost than Linux. Presumably the design choice has been made in
MySQL that since time is free [on Linux], it should be checked lots, which
tends to hurt MySQL performance more on non-Linux platforms. Some
experimental work has been done to offer reduced time quality via
alternative APIs, but we haven't settled on a particular approach.
Currently APIs don't allow you to say "I don't want to pay a lot for my
muf... timestamp.". I have a wrapper libc library somewhere that causes
applications to query lower quality time stamps using faster interfaces
which has been shown to have a significant impact on MySQL performance.

- MySQL seems to really like Linux threading and scheduling. For example, in
the past it has been observed that you get significantly better MySQL
performance on FreeBSD when you use the Linux threading implementation.
libthr closes some of that gap, but last I checked wasn't there yet. I'm
not really familiar with the issues here so can't speak to them in detail.

- Locking granularity has been an issue on SMP for MySQL on FreeBSD. The
patch I posted is part of working to address this. We have been seeing very
high contention on IPC locks in particular. With my change, Kris reports
much lower IPC lock contention -- instead we start hitting process-related
lock bottlenecks.

There's probably a lot more to this -- in the past there has also been speculation (although not, as far as I know, a lot of substantiation) that FreeBSD is running into some file system meta-data update bottlenecks, and that our sync interfaces for the file system are either more complete, less granular, or work too hard. Again, outside of my areas of expertise.

Part of the problem is probably an application optimization problem to do with optimizing MySQL to use features that are more cheap on Linux than FreeBSD, rather than vice versa -- there are many other applications that perform measurably better on FreeBSD than Linux, especially when hitting the VM, which suggests that while we probably do have problems we need to fix, something disportionate may have happened for MySQL performance. We do have significant overheads in our communications code that we've been working on actively, and differences in threading that need to be explored more.

Robert N M Watson
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