[openib-general] Re: Help with CONFIG_PCI_MSI in the kernel

[Michael S. Tsirkin]

Quoting r. Greg Lindahl <lindahl at pathscale.com>:
> Subject: Help with CONFIG_PCI_MSI in the kernel
> 
> Red Hat has started turning off CONFIG_PCI_MSI in their kernels (FC5
> and the latest FC4 update). I remember a while back there was a
> discussion about how MSI made the Mellanox HCA run faster, can someone
> please add some concrete details about this to the bug? Thanks.
> 
> http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=186520

Unfortunately its not a clear-cut win, or not on all systems.

At least for me, IRQ rebalancing seems to work much worse with MSI than with
regular interrupt messages: the interrupts seem to bounce between CPUs all the
time. I am only getting good performance from MSI by disabling IRQ rebalancing
and binding the task to a specific CPU.

It works fine with regular interrupts and I never had the time to get to the
bottom of it.

-- 
Michael S. Tsirkin
Staff Engineer, Mellanox Technologies