Linux patch increases multi-tasking performance

A couple days ago, a programmer named Mike Galbraith submitted a patch for the Linux kernel which significantly increases the performance of multi-tasking on the Linux desktop. With this patch, you can run multiple, CPU-intensive applications at the same time more effectively. Phoronix has two videos demonstrating the effect of this patch. Each video shows a computer that has number of tasks running simultaneously, such as compiling the Linux kernel and watching a high definition video. On the computer without the patch, the video was very stuttery and looked like a still image most of the time. But on the computer with the patch, the video plays almost perfectly. The patch is only about 200 lines of code, which is small compared to the large improvements the patch brings. Linus Torvalds, the developer in charge of the Linux kernel, liked the patch very much, calling it "one of those 'real improvement' patches", meaning that it creates few, if any, negative side-effects.

The patch achieves these performance gains by tweaking the scheduler (the scheduler determines when each running program is allowed to get CPU time). It works by grouping all programs by the TTY (terminal) that they were started in. CPU time is then spread out evenly across each group, as opposed to being spread out evenly across each individual program. Slashdot user tinkerghost gives a good example:

As an example, we need to run extinguish_fire and evacuate_building at the same time. extinguish_fire spawns a thread for each bucket in the brigade, while evacuate_building only spawns a thread for each escape route. Now, if there are 96 buckets and 4 escape routes, extinguish_fire will consume 96% of the CPU and choke out the evacuate_building threads...By grouping all of the threads from a program, extinguish_fire and evacuate_building get equal footing regardless of the number of threads they spawn.

Because it groups according to the TTY, it seems that you will only notice a performance improvement if you are running command-line programs. From what I've read, GUI programs are TTY-less. With this patch, GUI programs would all be lumped into the same group, so there would be no performance gain. In order to take advantage of the new scheduler, you would have to run a program from a new terminal in order to place it into another group.

Today, a Red Hat developer named Lennart Poettering found out that you can achieve the same thing just by running a few commands and editing a configuration file. This alternative solution is implemented differently, grouping programs by session instead of TTY, but it achieves the same effect. Linus says that this is actually how the kernel patch was originally tested, but that it should still be included in the kernel because it's such a good improvement.