The third major milestone release this year of the open-source Linux kernel is now available, providing users with new and improved features. The Linux 3.15 kernel follows Linux 3.14, which was formally released on March 30.
Among the enhanced capabilities in the Linux 3.15 kernel is improved suspend and resume performance. The new suspend and resume code “provides a tangible speed up for a non-esoteric use case (laptop resume),” Linux kernel developer Dan Williams told Linux creator Linus Torvalds in a Linux Kernel Mailing List (LKML) message.
Linux 3.15 packs a few key new features as well as a significant amount of “under-the-covers cleanup and restructuring,” Corbet said in an LWN article on the 3.15 merge window. Here’s a look at some of the highlights.
1. More Speed
Linux users across the board will likely appreciate a new feature in Linux 3.15 that allows systems with SATA disk controllers to resume from suspend much faster than they could before.
“Much of the time spent waiting for a system to resume goes into waiting for the ATA controllers to power up and get into a working state,” Corbet explained.
Now, with a pair of new patches, the drivers start the process immediately rather than waiting for the controller to return to a working state, allowing the rest of the kernel to continue working toward resuming the system while the controller powers up.
Bottom line? Resume time on a drive-heavy system dropped from 11.6 seconds to 1.1 seconds, Corbet noted; on single-drive systems, it went from more than five seconds to less than one.
“It is clearly a worthwhile improvement, especially since it requires little in the way of added complexity overall,” he said.
2. Better Performance
Of particular interest to users running workloads with large working sets is a patch set that improves memory management — specifically, the assessment of whether individual pages are being actively used or not.
Traditionally, Linux has maintained lists of both active and inactive pages, but it restricted the “active” list such that it couldn’t get longer than the “inactive” one, thereby causing problems. Now, through better balancing of those lists, performance can be improved, Corbet said.
Also improving performance in the new kernel is the fact that the FUSE (filesystems in user space) subsystem can now perform writeback caching, thus improving performance on write-heavy workloads. A per-thread virtual memory area (VMA) caching patch set, meanwhile, “should improve memory management performance for a number of workloads,” Corbet said.
3. Increased ARM Support
Support for a raft of new hardware has been added to Linux 3.15, including numerous ARM-based boards. Support for user-space probing with uprobes on ARM has been added as well.
The list of other freshly supported hardware includes several processors and systems, including two from Broadcom, as well as audio, graphics, networking equipment and more.
There are, of course, numerous other changes in Linux 3.15 as well. For a full summary, check out the changelog on KernelNewbies.org.
Linux 3.16 Up Next
With the Linux 3.15 release, Torvalds diverged somewhat from the long-established process by which new kernels are developed. Typically, the merge window for new code set to land in the Linux kernel does not open until the kernel that is currently in development is formally released. In the case of Linux 3.15, that would have meant that the merge window for the Linux 3.16 kernel would not open until June 9.
Due to Torvalds’ own vacation schedule and a desire not to be the reason for delay in the release of Linux 3.16, he actually opened up the merge window for Linux 3.16 on June 1.
“I’m not entirely convinced I liked the overlap, but it seemed to work ok, and unless people scream really loudly (‘Please don’t _ever_ do that again’) and give good reasons for doing so, I might end up doing that overlapping merge window in the future too if it ends up helping out with some particular timing issue,” Torvalds wrote in his release message for Linux 3.15. “That said, I also don’t think it was such a wonderful experience that I’d want to necessarily do the overlap every time, without a good specific reason for doing so.”