**Note: when I ran multiple threads on more cpus the performance
degradation phenomenon disappeared, but I tested on a VM with
qemu emulation backed by null_blk so I figured I had some other
bottleneck somewhere (that's why I asked for some more testing).
That could be because of the vmexits as every MMIO access in the guest
triggers a vmexit and if you poll with a low budget you do more MMIOs hence
you have more vmexits.
Did you do testing only in qemu or with real H/W as well?
I tried once. IIRC, I saw the same phenomenons...
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