Re: [libvirt-users] NUMA issues on virtualized hosts

Hello, so the current domain configuration: <cpu mode='host-passthrough'><topology sockets='8' cores='4' threads='1'/><numa><cell cpus='0-3' memory='62000000' /><cell cpus='4-7' memory='62000000' /><cell cpus='8-11' memory='62000000' /><cell cpus='12-15' memory='62000000' /><cell cpus='16-19' memory='62000000' /><cell cpus='20-23' memory='62000000' /><cell cpus='24-27' memory='62000000' /><cell cpus='28-31' memory='62000000' /></numa></cpu> <cputune><vcpupin vcpu='0' cpuset='0' /><vcpupin vcpu='1' cpuset='1' /><vcpupin vcpu='2' cpuset='2' /><vcpupin vcpu='3' cpuset='3' /><vcpupin vcpu='4' cpuset='4' /><vcpupin vcpu='5' cpuset='5' /><vcpupin vcpu='6' cpuset='6' /><vcpupin vcpu='7' cpuset='7' /><vcpupin vcpu='8' cpuset='8' /><vcpupin vcpu='9' cpuset='9' /><vcpupin vcpu='10' cpuset='10' /><vcpupin vcpu='11' cpuset='11' /><vcpupin vcpu='12' cpuset='12' /><vcpupin vcpu='13' cpuset='13' /><vcpupin vcpu='14' cpuset='14' /><vcpupin vcpu='15' cpuset='15' /><vcpupin vcpu='16' cpuset='16' /><vcpupin vcpu='17' cpuset='17' /><vcpupin vcpu='18' cpuset='18' /><vcpupin vcpu='19' cpuset='19' /><vcpupin vcpu='20' cpuset='20' /><vcpupin vcpu='21' cpuset='21' /><vcpupin vcpu='22' cpuset='22' /><vcpupin vcpu='23' cpuset='23' /><vcpupin vcpu='24' cpuset='24' /><vcpupin vcpu='25' cpuset='25' /><vcpupin vcpu='26' cpuset='26' /><vcpupin vcpu='27' cpuset='27' /><vcpupin vcpu='28' cpuset='28' /><vcpupin vcpu='29' cpuset='29' /><vcpupin vcpu='30' cpuset='30' /><vcpupin vcpu='31' cpuset='31' /></cputune> <numatune> <memnode cellid="0" mode="strict" nodeset="0"/> <memnode cellid="1" mode="strict" nodeset="1"/> <memnode cellid="2" mode="strict" nodeset="2"/> <memnode cellid="3" mode="strict" nodeset="3"/> <memnode cellid="4" mode="strict" nodeset="4"/> <memnode cellid="5" mode="strict" nodeset="5"/> <memnode cellid="6" mode="strict" nodeset="6"/> <memnode cellid="7" mode="strict" nodeset="7"/> </numatune> hopefully, I got it right.

Good news is, that spec benchmark looks promising. The first test bwaves finished in 1003 seconds compared to 1700 seconds in the previous wrong case. So far so good.

Bad news is, that iozone is still the same. There might be some misunderstanding. I have to cases: 1) cache=unsafe. In this case, I can see that hypervizor is prone to swap. Swap a lot. It usually eats whole swap partition and kswapd is running on 100% CPU. swappines, dirty_ration and company do not improve things at all. However, I believe, this is just wrong option for scratch disks where one can expect huge I/O load. Moreover, the hypevizor is poor machine with only low memory left (ok, in my case about 10GB available), so it does not make sense to use that memory for additional cache/disk buffers.

2) cache=none. In this case, performance is better (only few percent behind baremetal). However, as soon as the size of stored data is about the size of memory of the virtual, writes stops and iozone is eating whole CPU, it looks like it is searching more free pages and it is harder and harder. But not sure, I am not skilled in this area.

here, you can clearly see, that it starts writes, doing the writes, then it takes a pause, writes again, and so on, but the pauses are longer and longer.. https://pastebin.com/2gfPFgb9 The output is until the very end of iozone (I cancelled it by ctrl-c). It seems that this is not happening on 2-NUMA node with rotational disks only. It is partly happening on 2-NUMA node with 2 NVME SSDs. The partly means, that there are also pauses in writes but it finishes, speed is reduced though. On 1-NUMA node, with the same test, I can see steady writes from the very beginning to the very end at roughly the same speed. Maybe it could be related to the fact, that NVME is PCI device that is linked to one NUMA node only?

As of iothreads, I have only 1 disk (the vde) that is exposed to high i/o load, so I believe more I/O threads is not applicable here. If I understand correctly, I cannot set more iothreads to a single device.. And it does not seem to be iothreads linked as the same scenario in 1-NUMA configuration works OK (I mean that memory penalties can be huge as it does not reflect real NUMA topology, but disk speed it ok anyway.)