Messages in this thread | | | Subject | Re: [bug report] iommu_dma_unmap_sg() is very slow then running IO from remote numa node | From | Robin Murphy <> | Date | Wed, 28 Jul 2021 16:39:23 +0100 |
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On 2021-07-28 16:17, Ming Lei wrote: > On Wed, Jul 28, 2021 at 11:38:18AM +0100, John Garry wrote: >> On 28/07/2021 02:32, Ming Lei wrote: >>> On Mon, Jul 26, 2021 at 3:51 PM John Garry<john.garry@huawei.com> wrote: >>>> On 23/07/2021 11:21, Ming Lei wrote: >>>>>> Thanks, I was also going to suggest the latter, since it's what >>>>>> arm_smmu_cmdq_issue_cmdlist() does with IRQs masked that should be most >>>>>> indicative of where the slowness most likely stems from. >>>>> The improvement from 'iommu.strict=0' is very small: >>>>> >>>> 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.
Indeed, it now sounds like $SUBJECT has been a complete red herring, and although the SMMU may be reflecting the underlying slowness it is not in fact a significant contributor to it. Presumably perf shows any difference in CPU time moving elsewhere once iommu_dma_unmap_sg() is out of the picture?
>> 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/
Hmm, a ~4x discrepancy in CPU<->memory bandwidth is pretty significant, but it's still not the ~10x discrepancy in NVMe throughput. Possibly CPU<->PCIe and/or PCIe<->memory bandwidth is even further impacted between sockets, or perhaps all the individual latencies just add up - that level of detailed performance analysis is beyond my expertise. Either way I guess it's probably time to take it up with the system vendor to see if there's anything which can be tuned in hardware/firmware.
Robin.
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