There appears to be a performance regression going from 15.2.4 to HEAD.  I first realized this when testing my patches to Ceph on an 8-node cluster, but it is easily reproducible on *vanilla* Ceph with vstart as well, using the following steps:

$ git clone https://github.com/ceph/ceph.git && cd ceph

$ ./do_cmake.sh -DCMAKE_BUILD_TYPE=RelWithDebInfo -DWITH_MANPAGE=OFF -DWITH_BABELTRACE=OFF -DWITH_MGR_DASHBOARD_FRONTEND=OFF && cd build && make -j32 vstart

$ MON=1 OSD=1 MDS=0 ../src/vstart.sh --debug --new --localhost --bluestore --bluestore-devs /dev/xxx

$ sudo ./bin/ceph osd pool create foo 32 32

$ sudo ./bin/rados bench -p foo 100 write --no-cleanup

With the old hard drive that I have (Hitachi HUA72201), I'm getting an average throughput of 60 MiB/s.  When I switch to v15.2.4 (git checkout v15.2.4), rebuild, and repeat the experiment, and I get an average throughput of 90 MiB/s.  I've reliably reproduced similar difference between 15.2.4 and HEAD by building release packages and running them on an 8-node cluster.

Is this expected or is this a performance regression?

Thanks!