Sorry must be > > Hello, > > Doing some benchmark tests for my patch series I discovered that > direct I/O performance for a single thread onto raid456 md seems > to be not on par with what I would have expected. > > A simple 5 minute test run with md on top of /dev/ramX ramdisks > reveals: > > raid5 - 3 disks - chunk 32k - 1 writer > 4K random direct IO writes: 5MB/sec (five megabytes per second) > raid5 - 3 disks - chunk 32k - 1 writer 4K random writes (*NOT direct IO*): 600MB/sec > >raid5 - 3 disks - chunk 32k - 8 writers >4K random direct IO writes: 45MB/sec > raid5 - 3 disks - chunk 32k - 8 writers 4K random writes (*NOT direct IO*): 800MB/sec > > for comparison: > > raid1 - 3 disks - 1 writer > 4K random direct IO writes: 150MB/sec > > That would be a 1ms penalty per one md block write (or to be > precise per 3 device I/Os). A similar I/O overhead on the three > disk RAID1 shows reasonable numbers. > > A perf record of the 1 writer test gives: > > 38.40% swapper [kernel.kallsyms] [k] default_idle > 13.14% md0_raid5 [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore > 13.05% swapper [kernel.kallsyms] [k] tick_nohz_idle_enter > 10.01% iot [raid456] [k] raid5_unplug > 9.06% swapper [kernel.kallsyms] [k] tick_nohz_idle_exit > 3.39% md0_raid5 [kernel.kallsyms] [k] __kernel_fpu_begin > 1.67% md0_raid5 [xor] [k] xor_sse_2_pf64 > 0.87% iot [kernel.kallsyms] [k] finish_task_switch > > I'm confused and clueless. Especially I cannot see where the > 10% overhead in the source of raid5_unplug might come > from? Any idea from someone with better insight? > Markus
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