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

[Mike Snitzer]

On Thu, Feb 14 2019 at 11:54am -0500,
Mikulas Patocka <mpatocka at redhat.com> wrote:

> 
> 
> On Thu, 14 Feb 2019, Mike Snitzer wrote:
> 
> > 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().
> 
> We don't need to use mempools - if the slab allocation fails, we fall back 
> to the cloning path that has mempools.

But the implementation benefits from each DM device having control over
any extra memory it'd like to use for front padding.  Same as is done
now for the full-blown DM core with cloning.

> > 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
> 
> I tried to use per-bio-data and other features - and it makes the 
> structure dm_noclone and function noclone_endio grow:
> 
> #define DM_NOCLONE_MAGIC 9693664
> struct dm_noclone {
> 	struct mapped_device *md;
> 	struct dm_target *ti;
> 	struct bio *bio;
> 	struct bvec_iter orig_bi_iter;
> 	bio_end_io_t *orig_bi_end_io;
> 	void *orig_bi_private;
> 	unsigned long start_time;
> 	/* ... per-bio data ... */
> 	/* DM_NOCLONE_MAGIC */
> };
> 
> And this growth degrades performance on linear target - from 2238MiB/s to 
> 2145MiB/s.

It shouldn't if done properly.. for linear there wouldn't be any growth.