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Reduce taskq and context-switch cost of zio pipe
When doing a read from disk, ZFS creates 3 ZIO's: a zio_null(), the logical zio_read(), and then a physical zio. Currently, each of these results in a separate taskq_dispatch(zio_execute). On high-read-iops workloads, this causes a significant performance impact. By processing all 3 ZIO's in a single taskq entry, we reduce the overhead on taskq locking and context switching. We accomplish this by allowing zio_done() to return a "next zio to execute" to zio_execute(). This results in a ~12% performance increase for random reads, from 96,000 iops to 108,000 iops (with recordsize=8k, on SSD's). Reviewed by: Pavel Zakharov <email@example.com> Reviewed-by: Brian Behlendorf <firstname.lastname@example.org> Reviewed by: George Wilson <email@example.com> Signed-off-by: Matthew Ahrens <firstname.lastname@example.org> External-issue: DLPX-59292 Closes #7736
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