Re: [ata] 0568e61225: stress-ng.copy-file.ops_per_sec -15.0% regression

[John Garry <john.garry@huawei.com>]

On 12/08/2022 19:27, Damien Le Moal wrote:
>> Interestingly ata dev max_sectors kb also gets capped from 32MB (LBA48)
>> -> 256KB due to swiotlb max mapping size. (It would be capped by shost
>> default max sectors 512KB without that swiotlb limit). I assume capping
>> due to swiotlb limit is not occuring on Oliver's machine.
> Yes, I was suspecting that we may be seeing a difference for anything that is
> not AHCI, e.g. with other drivers.
> 
> But that seems to be the correct thing to do, no ? 
Yes, this should be the correct thing to do.

 > How was this working before
 > without applying the swiotlb limit ?
 >
Not sure. I would need to check the libata code for how it handles DMA 
mapping errors, which I assume would occur for when we exceed the 
swiotlb limit.

Having said this, from limited testing, whenever I check megaraid sas or 
mpt3sas for performance, the length of request data never/rarely comes 
close to max sectors. That why I am surprised with the regression which 
Oliver reports.

Thanks,
John