** Changed in: ubuntu-z-systems
Status: Incomplete => Triaged
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https://bugs.launchpad.net/bugs/1874062
Title:
[UBUNTU 20.04] Performance floating interrupt
Status in Ubuntu on IBM z Systems:
Triaged
Status in linux package in Ubuntu:
New
Bug description:
With the introduction of CPU directed interrupts the kernel parameter
pci=force_floating was introduced to fall back to the previous behavior using
floating irqs.
Furthermore this fallback is used on machines lacking support for directed
interrupts.
However we were still setting the affinity in that case, both in
__irq_alloc_descs() and via the irq_set_affinity callback in struct
irq_chip.
This setting of the affinity leads to a performance regression in
streaming workloads which can be seen with e.g. by an iperf streaming
test between two LPARs using ConnectX-5 based nics.
In some tests the performance would drop to 20% of the performance
with older kernels.
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