Re: [PATCH v3 0/5] dm: empty flush optimization

[YangYang]

On 2024/5/16 12:02, Yang Yang wrote:
> __send_empty_flush() sends empty flush bios to every target in the
> dm_table. However, if the num_targets exceeds the number of block
> devices in the dm_table's device list, it could lead to multiple
> invocations of __send_duplicate_bios() for the same block device.
> Typically, a single thread sending numerous empty flush bios to one
> block device is redundant, as these bios are likely to be merged by the
> flush state machine. In scenarios where num_targets significantly
> outweighs the number of block devices, such behavior may result in a
> noteworthy decrease in performance.
>
> This is a real-world scenario that we have encountered:
> 1) Call fallocate(file_fd, 0, 0, SZ_8G)
> 2) Call ioctl(file_fd, FS_IOC_FIEMAP, fiemap). In situations of severe
> file system fragmentation, fiemap->fm_mapped_extents may exceed 1000.
> 3) Create a dm-linear device based on fiemap->fm_extents
> 4) Create a snapshot-cow device based on the dm-linear device
>
> Perf diff of fio test:
>    fio --group_reporting --name=benchmark --filename=/dev/mapper/example \
>        --ioengine=sync --invalidate=1 --numjobs=16 --rw=randwrite \
>        --blocksize=4k --size=2G --time_based --runtime=30 --fdatasync=1
>
> Scenario one:
>    for i in {0..1023}; do
>      echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
>    done | sudo dmsetup create example
>
>    Before: bw=857KiB/
>    After:  bw=30.8MiB/s    +3580%
>
> Scenario two:
>    for i in {0..1023}; do
>      if [[ $i -gt 511 ]]; then
>        echo $((8000*$i)) 8000 linear /dev/nvme0n1p6 $((16384*$i))
>      else
>        echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
>      fi
>    done | sudo dmsetup create example
>
>    Before: bw=1470KiB/
>    After:  bw=33.9MiB/s    +2261%
>
> Any comments are welcome!
>
> V3:
> -- Focus on targets with num_flush_bios equal to 1 to simplify the code
> -- Use t->devices_lock to protect the dm_table's device list

Please ignore V3, which has a build warning. I will send V4 with the fix.

Thanks