Re: [bug report] iommu_dma_unmap_sg() is very slow then running IO from remote numa node

[John Garry]

On 28/07/2021 16:17, Ming Lei wrote:
> > > > Have you tried turning off the IOMMU to ensure that this is really just
> > > > an IOMMU problem?
> > > >
> > > > You can try setting CONFIG_ARM_SMMU_V3=n in the defconfig or passing
> > > > cmdline param iommu.passthrough=1 to bypass the the SMMU (equivalent to
> > > > disabling for kernel drivers).
> > > Bypassing SMMU via iommu.passthrough=1 basically doesn't make a difference
> > > on this issue.
> > A ~90% throughput drop still seems to me to be too high to be a software
> > issue. More so since I don't see similar on my system. And that throughput
> > drop does not lead to a total CPU usage drop, from the fio log.
> >
> > Do you know if anyone has run memory benchmark tests on this board to find
> > out NUMA effect? I think lmbench or stream could be used for this.
> https://lore.kernel.org/lkml/YOhbc5C47IzC893B@T590/

Hi Ming,

Out of curiosity, did you investigate this topic any further?

And you also asked about my results earlier:

On 22/07/2021 16:54, Ming Lei wrote:
>> [   52.035895] nvme 0000:81:00.0: Adding to iommu group 5
>> [   52.047732] nvme nvme0: pci function 0000:81:00.0
>> [   52.067216] nvme nvme0: 22/0/2 default/read/poll queues
>> [   52.087318]  nvme0n1: p1
>>
>> So I get these results:
>> cpu0 335K
>> cpu32 346K
>> cpu64 300K
>> cpu96 300K
>>
>> So still not massive changes.
> In your last email, the results are the following with irq mode io_uring:
>
>   cpu0  497K
>   cpu4  307K
>   cpu32 566K
>   cpu64 488K
>   cpu96 508K
>
> So looks you get much worse result with real io_polling?
>

Would the expectation be that at least I get the same performance with 
io_polling here? Anything else to try which you can suggest to 
investigate this lower performance?

Thanks,
John