On 11.12.24 18:50, Niklas Schnelle wrote:
> On Wed, 2024-12-11 at 18:28 +0100, Dragos Tatulea wrote:
>>>>>>
>
> ---8<---
>
>>
>>> On 09.12.24 12:36, Tariq Toukan wrote:
>>>> Hi,
>>>>
>>>> Many approaches in the past few years are going the opposite direction, trying to avoid copies ("zero-copy").
>>>>
>>>> In many cases, copy up to PAGE_SIZE means copy everything.
>>>> For high NIC speeds this is not realistic.
>>>>
>>>> Anyway, based on past experience, threshold should not exceed "max header size" (128/256b).
>>>
>>>>> 1KB is still to large. As Tariq mentioned, the threshold should not
>>>>> exceed 128/256B. I am currently testing this with 256B on x86. So far no
>>>>> regressions but I need to play with it more.
>> I checked on a x86 system with CX7 and we seem to get ~4% degradation
>> when using this approach. The patch was modified a bit according to
>> previous discussions (diff at end of mail).
>>
>> Here's how I tested:
>> - uperf client side has many queues.
>> - uperf server side has single queue with interrupts pinned to a single
>> CPU. This is to better isolate CPU behaviour. The idea is to have the
>> CPU on the server side saturated or close to saturation.
>> - Used the uperf 1B read x 1B write scenario with 50 and 100 threads
>> (profile attached).
>> Both the on-cpu and off-cpu cases were checked.
>> - Code change was done only on server side.
>
> I'm assuming this is with the x86 default IOMMU pass-through mode?
It was in a VM with PCI passthrough for the device.
> Would you be able and willing to try with iommu.passthrough=0
> and amd_iommu=on respectively intel_iommu=on? Check
> /sys/bus/pci/devices/<dev>/iommu_group/type for "DMA-FQ" to make sure
> the dma-iommu code is used. This is obviously not a "tuned for all out
> perf at any cost" configuration but it is recommended in hardening
> guides and I believe some ARM64 systems also default to using the IOMMU
> for bare metal DMA API use. So it's not an unexpected configuration
> either.
>
I got hold of a bare metal system where I could turn iommu passthrough
off and confirm iommu_group/type as being DMA_FQ. But the results are
inconclusive due to instability. I will look into this again after the
holidays.
Thanks,
Dragos
