One week ago, kernel hacker Ingo Molnar reviewed Con Kolivas’ swap prefetch patches and approved them to be added to the official Linux kernel. Swap prefetching is a technique, which will load swapped out memory pages back in memory if the system is idle and memory has become available. This is useful for people starting temporary jobs which use a lot of memory, which makes other processes move to swap. Once the memory hungry process is finished, swap prefetching will kick in and slowly reload the swapped out pages back to memory, so that the system potentially does not need to do this anymore when the user again uses one of the swapped out processes. Tjos functionality can be enabled and disabled at will during compilation of the kernel.
Swap prefetching has been available for a long time in Con Kolivas patch set, and was also added to the mm development kernels some time ago. In that period, no bugs have been reported, and it seems people are happy with this feature. So it was hoped that the push by Ingo Molnar, would finally make swap prefetching available for all Linux users in version 2.6.22.
Developer Nick Piggin seemed rather critical of the swap prefetching feature. I I have understood the thread correctly, he discovered a problem (actually swap prefetching did not seem to work anymore because of some unrelated changes in Linux), and there was some serious disagreement how it should be dealt with. Con Kolivas’ got fed up with the unreasonable objections to the patch and proposed to dump the patch completely. So this is yet another performance improving feature we won’t be seeing in the Linux kernel for some time…
Update 12 May 2007: Con Kolivas posted an updated version of swap prefetch, addressing some problems, and the patch has not been dropped from the mm kernel. Maybe things do not look so bad after all?
This whole story makes me remind of the recent RSDL/SD scheduler. Con Kolivas implements a feature which is clearly working very good for most people, and then someone comes up who vetoes it for whatever reason, and in the case of SD other implementations are started (which still do not have the same maturity as SD). In the end a lot of work seems to duplicated and interesting features are delayed or even cancelled completely. Is it me, or is the behaviour of some kernel developers really hurting Linux development?
Also in related news, kernel hacker Adrian Bunk decided to not track regressions anymore. He was tired of Linux 2.6.21 being released with lots of known regressions. It seems Linux kernel development has really some issues these days…
Today kernel developer Con Kolivas announced that he will stop developing his Linux patch which improves desktop performance. For people who have followed recent discussions about his SD CPU scheduler and about the inclusion of his swap prefetching patche