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Date:   Mon, 11 Jul 2022 08:36:53 +0100
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Subject: Re: [PATCH v5 0/5] DMA mapping changes for SCSI core
To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Christoph Hellwig <hch@lst.de>
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From:   John Garry <john.garry@huawei.com>
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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