A Set Of BFQ Improvements Ready For Testing
Written by Michael Larabel in Linux Kernel on 22 September 2017 at 06:01 AM EDT. 1 Comment
LINUX KERNEL --
Recently I wrote about a BFQ regression fix that should take care of a problem spotted in our recent I/O scheduler Linux 4.13 benchmarks while now that work has yielded a set of four patches working to improve this recently-merged scheduler.

Paolo Valente of Linaro sent out a set of fixes for affecting service guarantees.

He explained in the patch series:
the first patch in this series fixes a bug that causes bfq to fail to guarantee a high responsiveness on some drives, if there is heavy random read+write I/O in the background. More precisely, such a failure allowed this bug to be found [1], but the bug may well cause other yet unreported anomalies.

This fix uncovered other bugs that were concealed by the fixed bug, for rather subtle reasons. These further bugs caused similar responsiveness failures, but with sequential reaad+write workloads in the background. The remaining three patches fix these further bugs.

The sum of these fixes makes responsiveness much stabler with BFQ. In the presence of write hogs, it is however still impossible for an I/O scheduler to guarantee perfect responsiveness in any circumstance, because of throttling issues in the virtual-memory management subsystem, and in other higher-level components.
The work for now is on the kernel mailing list. It isn't clear at the moment if Paolo will try to get these into Linux 4.14 or wait until 4.15. Anyhow, I'll have some new I/O scheduler tests shortly on Phoronix.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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