Open Stack

Thanks for the fast reply and for sharing your experience.

We have considered removing isolcpus as well but the idea of
introducing noise into guest workload is somewhat concerning. Also
restraining dockerized deployment without isolcpus will not be as
easy. We definitely keep this option as a last resort.

On Wed, 28 Dec 2022 at 15:59, Ümit Seren <uemit.seren at gmail.com> wrote:
>
> We had a similar performance issue with networking  (via openswitch) instead of I/O.
>
> Our hypervisor and VM configuration were like yours (VCPU pinning + isolcpus). We saw a 50% drop in virtualized networking throughput (measure via iperf).
> This was because the vhost_net kthreads which are responsible for the virtualized networking were pinned to 2 cores per socket and this quickly became the bottleneck. This was with OpenStack Queens and RHEL 7.6.
>
> We ended up keeping the VCPU pinning but removing the isolcpus kernel setting. This fixed the performance regression.
>
>
>
> Unfortunately, we didn’t further investigate this, so I don’t know why a newer kernel and/or newer Openstack release improves it.
>
>
>
> Hope this still helps
>
>
>
> Best
>
> Ümit
>
>
>
> On 27.12.22, 13:33, "Can Özyurt" <acozyurt at gmail.com> wrote:
>
> 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.
>
>
>
>
>
>