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

[Damien Le Moal]

On 2022/08/10 1:33, John Garry wrote:
> On 09/08/2022 15:57, Damien Le Moal wrote:
>>>> As far as I can see, this patch should not make a difference unless the
>>>> ATA shost driver is setting the max_sectors value unnecessarily low.
>>> For __ATA_BASE_SHT, we don't set max_sectors. As such, we default
>>> shost->max_sectors = SCSI_DEFAULT_MAX_SECTORS (=1024) in
>>> scsi_host_alloc(). I assume no shost dma mapping limit applied.
>>>
>>> Then - for example - we could select dev->max_sectors =
>>> ATA_MAX_SECTORS_LBA48 (=65535) in ata_dev_configure().
>>>
>>> So with commit 0568e6122574 we would have final max sectors = 1024, as
>>> opposed to 65535 previously. I guess that the problem is something like
>>> this.
>>>
>>> If so, it seems that we would need to apply the shost dma mapping limit
>>> separately in ata_scsi_dev_config() and not use shost->max_sectors.
>> OK. Will have a look at that.
>>
> 
> We may need to introduce something like shost->max_hw_sectors, which is 
> set according to sht max sectors and dma mapping limits. That could be 
> also used in USB scsiglue slave_configure()
> 
> Or else set max_sectors value for __ATA_BASE_SHT, but I don't know a 
> sane value there considering ATA_MAX_SECTORS_LBA48 gives max_sectors of 
> 65535.
> 
> Damien, please let me know if you need help now. I am just waiting for 
> you to test to prove this theory about dev->max_sectors being capped. I 
> don't have an AHCI setup readily-available for testing - just SAS cards 
> or QEMU.

I am on it.

> 
> Thanks,
> John


-- 
Damien Le Moal
Western Digital Research