Messages in this thread | | | Date | Mon, 11 Jul 2022 08:36:53 +0100 | Subject | Re: [PATCH v5 0/5] DMA mapping changes for SCSI core | From | John Garry <> |
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On 11/07/2022 00:08, Damien Le Moal wrote: >> Ah, I think that I misunderstood Damien's question. I thought he was >> asking why not keep shost max_sectors at dma_max_mapping_size() and then >> init each sdev request queue max hw sectors at dma_opt_mapping_size(). > I was suggesting the reverse:) Keep the device hard limit > (max_hw_sectors) to the max dma mapping and set the soft limit > (max_sectors) to the optimal dma mapping size.
Sure, but as I mentioned below, I only see a small % of requests whose mapping size exceeds max_sectors but that still causes a big performance hit. So that is why I want to set the hard limit as the optimal dma mapping size.
Indeed, the IOMMU IOVA caching limit is already the same as default max_sectors for the disks in my system - 128Kb for 4k page size.
> >> But he seems that you want to know why not have the request queue max >> sectors at dma_opt_mapping_size(). The answer is related to meaning of >> dma_opt_mapping_size(). If we get any mappings which exceed this size >> then it can have a big dma mapping performance hit. 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. >>
Thanks, John
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