On ext4 filesystems, ext4-dio-unwritten workqueue should be single threaded instead of one thread per CPU.

Solution Unverified - Updated -

Issue

  • On ext4 filesystems, ext4-dio-unwritten workqueue should be single threaded instead of one thread per CPU.

  • If users isolate a realtime application to a single core (to benefit from cache coherency/cache hit), then if the ext4-dio-unwritten threads also process conversions from unwritten extents to written for IOs completed from async direct IO path, the application gets a performance hit.

  • Per filesystem thread to write unwritten extent to written IO is enough, we don't need per cpu threads to work on conversion.

Environment

  • Red Hat Enterprise Linux (RHEL) 6 up to RHEL6.4.

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