** Description changed:
- Description will follow
+ 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.
** Also affects: ubuntu-z-systems
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1874062
Title:
[UBUNTU 20.04] Performance floating interrupt
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