On 2022/08/09 8:16, David Laight wrote: > ... >>>> 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. >> >> That is my hunch too, hence my question about which host driver is being used >> for this test... That is not apparent from the problem report. > > No one's fallen over the old problem and managed to limit > the number of sectors in a read to the number of sectors > in (IIRC) a 'multi sector' read that uses a single DMA burst? > > Was always a good way of killing disk performance. > > IIRC the maximum number of sectors for an ATA disk transfer is 255. That is for super old pata/ide devices. Modern SATA/AHCI disks can do a lot more than that with NCQ/DMA. The default max_sectors_kb for AHCI connected devices is 1280 KB = 2560 sectors, max_segments = 168 and max_segment_size = 65536 B, which gives a reliable command size of at least 168 * 4096 = 688128 B (1344 sectors) with 4KB page size architectures. > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) -- Damien Le Moal Western Digital Research
