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60 • Linux Symposium 2004 • Volume One plications. 3 Linux-2.7 The 2.5 development cycle saw the most massively changed block layer in the history of Linux. Before 2.5 was opened, Linus had clearly expressed that one of the most important things that needed doing, was the block layer update. And indeed, the very first thing merged was the complete bio patch into 2.5.1- pre2. At that time, no more than a handful drivers compiled (let alone worked). The 2.7 changes will be nowhere as severe or drastic. A few of the possible directions will follow in the next few sections. 3.1 IO Priorities Prioritized IO is a very interesting area that is sure to generate lots of discussion and development. It’s one of the missing pieces of the complete resource management puzzle that several groups of people would very much like to solve. People running systems with many users, or machines hosting virtual hosts (or completed virtualized environments) are dying to be able to provide some QOS guarantees. Some work was already done in this area, so far nothing complete has materialized. The CKRM [CKRM] project spear headed by IBM is an attempt to define global resource management, including io. They applied a little work to the CFQ IO scheduler to provide equal bandwidth between resource management classes, but at no specific priorities. Currently I have a CFQ patch that is 99% complete that provides full priority support, using the IO contexts introduced by AS to manage fair sharing over the full time span that a process exists 7 . This works well enough, but only works 7 CFQ currently tears down class structures as soon as it is empty, it doesn’t persist over process life time. for that specific IO scheduler. A nicer solution would be to create a scheme that works independently of the io scheduler used. That would require a rethinking of the IO scheduler API. 3.2 IO Scheduler switching Currently Linux provides no less than 4 IO schedulers—the 3 mentioned, plus a forth dubbed noop. The latter is a simple IO scheduler that does no request reordering, no latency management, and always merges whenever it can. Its area of application is mainly highly intelligent hardware with huge queue depths, where regular request reordering doesn’t make sense. Selecting a specific IO scheduler can either be done by modifying the source of a driver and putting the appropriate calls in there at queue init time, or globally for any queue by passing the elevator=xxx boot parameter. This makes it impossible, or at least very impractical, to benchmark different IO schedulers without many reboots or recompiles. Some way to switch IO schedulers per queue and on the fly is desperately needed. Freezing a queue and letting IO drain from it until it’s empty (pinning new IO along the way), and then shutting down the old io scheduler and moving to the new scheduler would not be so hard to do. The queues expose various sysfs variables already, so the logical approach would simply be to: # echo deadline > \ /sys/block/hda/queue/io_scheduler A simple but effective interface. At least two patches doing something like this were already proposed, but nothing was merged at that time. 4 Final comments The block layer code in 2.6 has come a long way from the rotted 2.4 code. New features
Linux Symposium 2004 • Volume One • 61 bring it more up-to-date with modern hardware, and completely rewritten from scratch core provides much better scalability, performance, and memory usage benefiting any machine from small to really huge. Going back a few years, I heard constant complaints about the block layer and how much it sucked and how outdated it was. These days I rarely hear anything about the current state of affairs, which usually means that it’s doing pretty well indeed. 2.7 work will mainly focus on feature additions and driver layer abstractions (our concept of IDE layer, SCSI layer etc will be severely shook up). Nothing that will wreak havoc and turn everything inside out like 2.5 did. Most of the 2.7 work mentioned above is pretty light, and could easily be back ported to 2.6 once it has been completed and tested. Which is also a good sign that nothing really radical or risky is missing. So things are settling down, a sign of stability. [Higdon] Jeremy Higdon, Re: [PATCH] per-backing dev unplugging #2, Linux kernel mailing list http://marc.theaimsgroup. com/?l=linux-kernel&m= 107941470424309&w=2, 2004 [CKRM] IBM, Class-based Kernel Resource Management (CKRM), http://ckrm.sf.net, 2004 [Bhattacharya] Suparna Bhattacharya, Notes on the Generic Block Layer Rewrite in Linux 2.5, General discussion, Documentation/block/biodoc. txt, 2002 References [Iyer] Sitaram Iyer and Peter Druschel, Anticipatory scheduling: A disk scheduling framework to overcome deceptive idleness in synchronous I/O, 18th ACM Symposium on Operating Systems Principles, http: //www.cs.rice.edu/~ssiyer/ r/antsched/antsched.ps.gz, 2001 [McKenney] Paul E. McKenney, Stochastic Fairness Queuing, INFOCOM http: //rdrop.com/users/paulmck/ paper/sfq.2002.06.04.pdf, 1990 [Chen] Kenneth W. Chen, per-cpu blk_plug_list, Linux kernel mailing list http://www.ussg.iu.edu/ hypermail/linux/kernel/ 0403.0/0179.html, 2004
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Proceedings of the Linux Symposium
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The State of ACPI in the Linux Kern
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Conference Organizers Andrew J. Hut
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Linux Virtualization on IBM POWER5
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The State of ACPI in the Linux Kern
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Scaling Linux® to the Extreme From
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Comparing and Evaluating epoll, sel
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The (Re)Architecture of the X Windo
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IA64-Linux perf tools for IO dorks
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Hotplug Memory and the Linux VM Dav
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