Messages in this thread | | | Date | Thu, 14 Jul 2022 08:52:15 +0100 | Subject | Re: [PATCH v5 0/5] DMA mapping changes for SCSI core | From | John Garry <> |
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On 14/07/2022 04:10, Martin K. Petersen wrote:
Hi Martin,
>> So I set max hw sectors at this ‘opt’ mapping size to ensure that we >> get no mappings which exceed this size. Indeed, I think max sectors is >> 128Kb today for my host, which would be same as dma_opt_mapping_size() >> value with an IOMMU enabled. And I find that only a small % of request >> size may exceed this 128kb size, but it still has a big performance >> impact. > The purpose of the soft limit is to pick the appropriate I/O size > (i.e. for best performance). The purpose of the hard limit is to ensure > we don't submit something the hardware can't handle or describe. > > IOW, the hard limit is not about performance at all. The hard limit is > mainly relevant for things that are way bigger than anything we'd issue > as regular filesystem I/O such as multi-megabyte firmware images, etc. > > It's perfectly fine for firmware download performance to be > "suboptimal". What is typically more important in that scenario is that > the firmware image makes it inside a single I/O.
OK, fine. I've improved the next version such that the DMA mapping opt limit only affects the max_sectors default.
Thanks, John
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