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

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 2022/08/09 7:16, John Garry wrote:
> On 09/08/2022 10:58, John Garry wrote:
>>>>
>>>> commit: 0568e6122574dcc1aded2979cd0245038efe22b6 ("ata: libata-scsi: 
>>>> cap ata_device->max_sectors according to shost->max_sectors")
>>>> https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
>>>>
>>>> in testcase: stress-ng
>>>> on test machine: 96 threads 2 sockets Ice Lake with 256G memory
>>>> with following parameters:
>>>>
>>>>     nr_threads: 10%
>>>>     disk: 1HDD
>>>>     testtime: 60s
>>>>     fs: f2fs
>>>>     class: filesystem
>>>>     test: copy-file
>>>>     cpufreq_governor: performance
>>>>     ucode: 0xb000280
>>>
>>> Without knowing what the device adapter is, hard to say where the 
>>> problem is. I
>>> suspect that with the patch applied, we may be ending up with a small 
>>> default
>>> max_sectors value, causing overhead due to more commands than necessary.
>>>
>>> Will check what I see with my test rig.
>>
>> 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.

> 
> thanks,
> John
> 


-- 
Damien Le Moal
Western Digital Research