GNU C Library 2.19 and what developers can get from it

The GNU C Library project released version 2.19 of it’s library on Saturday (Friday for some), with Allan McRae as the release manager. Apart from numerous bug fixes, there are a couple improvements that would interest developers. Both improvements are related in some manner to the library documentation, which apparently is not very well known. In fact, it seems that very few people know that the official documentation for functions in the GNU C library is not the man pages, it is the GNU C Library Manual. This is not to discredit the man page project in any way of course - Michael Kerrisk does a swell job of keeping man pages in sync with glibc behaviour wherever necessary and I’d like to think that we’re cooperating to the best of our abilities. The man page project however is more general and covers a fairly broad set of components including the kernel, the C library and some tools. The glibc manual focusses specifically on functionality provided by glibc.

Now for the first big improvement. Alexandre Oliva, along with a number of reviewers did the mammoth job of adding documentation regarding multi-thread safety, async-signal safety and async-cancellation safety for functions provided by glibc. This is an invaluable resource because it tries to describe precisely what kind of guarantees the glibc implementation of various functions provides, as opposed to the guarantees documented in the various standards.

The second improvement is the introduction of Systemtap userspace probe markers for various events in malloc and some mathematical functions. The malloc subsystem is fairly complex and has some critical events that a developer may want to track when trying to profile memory allocation patterns for their programs. These probes are placed at such critical points in the malloc subsystem so that one may write up a systemtap script to profile their applications much more easily. I had written a description of the malloc internal implementation some time ago, which is still relevant and may help developers select the appropriate probes.

Some mathematical functions try to provide a guarantee of accuracy of the result to the last bit and to do so, some inputs may require multiple precision computation (I have of course, written about this in a bit more detail in the past). This fallback computation may be in multiple stages and may take anywhere between a 100 times to a 1000 times more than the normal execution speed of the function. While this fallback is needed only for a handful of inputs in the entire range, the performance impact is really high when an application does hit this path. So to help developers identify whether their performance hit is due to these multiple precision fallback paths, these paths have been marked with probe markers that can be used in systemtap scripts to profile applications. The probes have been documented in the libc manual, in the Internal Probes section.

Finally, I have managed to finish producing a significant set of benchmark inputs for the math functions I care about, so it might be a good time for folks to start trying them out and sending in results. The README in the benchtests directory should be a good starting point. The output file format is still not final - I’m toying with JSON as the final format - so expect changes there in future. The string benchmarks still need some attention, which hopefully will happen in the 2.20 time frame.

Looking forward to 2.20, Joseph Myers has already begun the work of moving the architectures in the ports directory to the regular source tree. Once this is complete, we will have no concept of secondary architectures, which is a good thing. Hopefully in future we will also get rid of the libc-ports mailing list, which I have complained about in the past as being an unnecessary separation.

On the benchmarks front, we’ll be moving to python as the language of choice for the scripts and adding features such as graphing, a better file format, scripts to compare benchmark outputs and anything else that catches my fancy during the next 6 months.

Finally, I’ve mentioned this before - The GNU C Library manual needs contributors and here’s how you can help.

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