On Fri, 2016-06-24 at 14:29 -0400, Mike Snitzer wrote:
>
> BTW, if in your testing you could evaluate/quantify any extra overhead
> from DM that'd be useful to share. It could be there are bottlenecks
> that need to be fixed, etc.
Here are some results from fio benchmark. The test is single-threaded and is
bound to one CPU.
DAX LVM IOPS NOTE
---------------------------------------
Y N 790K
Y Y 754K 5% overhead with LVM
N N 567K
N Y 457K 20% overhead with LVM
DAX: Y: mount -o dax,noatime, N: mount -o noatime
LVM: Y: dm-linear on pmem0 device, N: pmem0 device
fio: bs=4k, size=2G, direct=1, rw=randread, numjobs=1
Among the 5% overhead with DAX/LVM, the new DM direct_access interfaces
account for less than 0.5%.
dm_blk_direct_access 0.28%
linear_direct_access 0.17%
The average latency increases slightly from 0.93us to 0.95us. I think most of
the overhead comes from the submit_bio() path, which is used only for
accessing metadata with DAX. I believe this is due to cloning bio for each
request in DM. There is 12% more L2 miss in total.
Without DAX, 20% overhead is observed with LVM. Average latency increases
from 1.39us to 1.82us. Without DAX, bio is cloned for both data and metadata.
Thanks,
-Toshi
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