On 7/11/22 16:36, John Garry wrote: > 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. How can you possibly end-up with requests larger than max_sectors ? BIO split is done using this limit, right ? Or is it that request merging is allowed up to max_hw_sectors even if the resulting request size exceeds max_sectors ? > > 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 -- Damien Le Moal Western Digital Research
