Open Stack

Hi everyone,

I hope you are all doing well. We are trying to pinpoint an IOPS
problem with RBD and decided to ask you for your take on it.

1 control plane
1 compute node
5 storage with 8 SSD disks each
Openstack Stein/Ceph Mimic deployed with kolla-ansible on ubuntu-1804
(kernel 5.4)
isolcpus 4-127 on compute
vcpu_pin_set 4-127 in nova.conf

image_metadatas:
  hw_scsi_model: virtio-scsi
  hw_disk_bus: scsi
flavor_metadatas:
  hw:cpu_policy: dedicated

What we have tested:
fio --directory=. --ioengine=libaio --direct=1
--name=benchmark_random_read_write --filename=test_rand --bs=4k
--iodepth=32 --size=1G --readwrite=randrw --rwmixread=50 --time_based
--runtime=300s --numjobs=16

1. First we run the fio test above on a guest VM, we see average 5K/5K
read/write IOPS consistently. What we realize is that during the test,
one single core on compute host is used at max, which is the first of
the pinned cpus of the guest. 'top -Hp $qemupid' shows that some
threads (notably tp_librbd) share the very same core throughout the
test. (also emulatorpin set = vcpupin set as expected)
2. We remove isolcpus and every other configuration stays the same.
Now fio tests now show 11K/11K read/write IOPS. No bottlenecked single
cpu on the host, observed threads seem to visit all emulatorpins.
3. We bring isolcpus back and redeploy the cluster with Train/Nautilus
on ubuntu-1804. Observations are identical to #1.
4. We tried replacing vcpu_pin_set to cpu_shared_set and
cpu_dedicated_set to be able to pin emulator cpuset to 0-4 to no
avail. Multiple guests on a host can easily deplete resources and IOPS
drops.
5. Isolcpus are still in place and we deploy Ussuri with kolla-ansible
and Train (to limit the moving parts) with ceph-ansible both on
ubuntu-1804. Now we see 7K/7K read/write IOPS.
6. We destroy only the compute node and boot it with ubuntu-2004 with
isolcpus set. Add it back to the existing cluster and fio shows
slightly above 10K/10K read/write IOPS.


What we think happens:

1. Since isolcpus disables scheduling between given cpus, qemu process
and its threads are stuck at the same cpu which created the
bottleneck. They should be runnable on any given emulatorpin cpus.
2. Ussuri is more performant despite isolcpus, with the improvements
made over time.
3. Ubuntu-2004 is more performant despite isolcpus, with the
improvements made over time in the kernel.

Now the questions are:

1. What else are we missing here?
2. Are any of those assumptions false?
3. If all true, what can we do to solve this issue given that we
cannot upgrade openstack nor ceph on production overnight?
4. Has anyone dealt with this issue before?

We welcome any opinion and suggestions at this point as we need to
make sure that we are on the right path regarding the problem and
upgrade is not the only solution. Thanks in advance.