[dm-devel] [PATCH 4/4] dm: implement no-clone optimization

[Mike Snitzer]

On Thu, Feb 14 2019 at 10:00am -0500,
Mikulas Patocka <mpatocka at redhat.com> wrote:

> This patch improves performance of dm-linear and dm-striped targets.
> Device mapper copies the whole bio and passes it to the lower layer. This
> copying may be avoided in special cases.
> 
> This patch changes the logic so that instead of copying the bio we
> allocate a structure dm_noclone (it has only 4 entries), save the values
> bi_end_io and bi_private in it, overwrite these values in the bio and pass
> the bio to the lower block device.
> 
> When the bio is finished, the function noclone_endio restores te values
> bi_end_io and bi_private and passes the bio to the original bi_end_io
> function.
> 
> This optimization can only be done by dm-linear and dm-striped targets,
> the target can op-in by setting ti->no_clone = true.
> 
> Performance improvement:
> 
> # modprobe brd rd_size=1048576
> # dd if=/dev/zero of=/dev/ram0 bs=1M oflag=direct
> # dmsetup create lin --table "0 2097152 linear /dev/ram0 0"
> # fio --ioengine=psync --iodepth=1 --rw=read --bs=512 --direct=1 --numjobs=12 --time_based --runtime=10 --group_reporting --name=/dev/mapper/lin
> 
> x86-64, 2x six-core
> /dev/ram0					2449MiB/s
> /dev/mapper/lin 5.0-rc without optimization	1970MiB/s
> /dev/mapper/lin 5.0-rc with optimization	2238MiB/s
> 
> arm64, quad core:
> /dev/ram0					457MiB/s
> /dev/mapper/lin 5.0-rc without optimization	325MiB/s
> /dev/mapper/lin 5.0-rc with optimization	364MiB/s
> 
> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

Nice performance improvement.  But each device should have its own
mempool for dm_noclone + front padding.  So it should be wired into
dm_alloc_md_mempools().

It is fine if you don't actually deal with supporting per-bio-data in
this patch, but a follow-on patch to add support for noclone-based
per-bio-data shouldn't be expected to refactor the location of the
mempool allocation (module vs per-device granularity).

Mike