On 7/26/22 05:54, Avri Altman wrote:
Bart hi,
Measurements have shown that for some UFS devices the maximum sequential
I/O throughput is achieved with a transfer size above 512 KiB. Hence
increase the maximum size of the data buffer associated with a single
request from SCSI_DEFAULT_MAX_SECTORS (1024) * 512 bytes = 512 KiB into
255 MiB.
Notes:
- The maximum data buffer size supported by the UFSHCI specification
is 65535 * 256 KiB or about 16 GiB.
- The maximum data buffer size for READ(10) commands is 65535 logical
blocks. To transfer more than 65535 * 4096 bytes = 255 MiB with a
single SCSI command, the READ(16) command is required. Support for
READ(16) is optional in the UFS 3.1 and UFS 4.0 standards.
I still have concerns of a negative impact of a too-large-max-sectors.
I replicate your fio measurements using galaxy S22 - one of the most advanced production platform currently in the market.
See the results below: fio bs vs max-sectors. data is the read BW[MiB/s].
Given that:
a) there isn't that much of a difference among the various max-sectors
b) max-sectors is configurable via max_sectors_kb sysfs entry, and
c) going from the benchmark world into the real world, a large write (255MiB),
interleaved by many small reads (4k), may cause high latency, up to even a timeout (!?)
I would leave the max-sectors threshold as it is, or use the moderate 1MB limit.
Unless you can back the "Measurements have shown" statement with a more concrete data.
Thanks,
Avri
| 1MB | 2MB | 128MB | 255MB
--------------------------------------------------------------------------------------------------------------------
4KiB | 14.7 | 13.3 | 15.7 | 19.5
8KiB | 26.7 | 27.4 | 25.8 | 30.0
16KiB | 44 | 53.2 | 46.3 | 61.2
64KiB | 179 | 207 | 171 | 184
32KiB | 100 | 120 | 105 | 119
128KiB | 275 | 322 | 271 | 256
256KiB | 440 | 452 | 417 | 378
512KiB | 535 | 525 | 483 | 471
1MiB | 592 | 584 | 531 | 408
2MiB | 349 | 361 | 375 | 297
4MiB | 502 | 492 | 515 | 441
8MiB | 645 | 663 | 682 | 650
16MiB | 752 | 749 | 752 | 762
32MiB | 817 | 790 | 822 | 791
64MiB | 1710 | 1703 | 1700 | 1692
128MiB | 1733 | 1730 | 1724 | 1712
256MiB | 1752 | 1748 | 1738 | 1732
Hi Avri,
Thank you for having run these measurements. Regarding (b): the
max_sectors_kb sysfs entry can be used to reduce the value set by a SCSI
LLD but not to increase that value. From block/blk-sysfs.c:
if (max_sectors_kb > max_hw_sectors_kb)
return -EINVAL;
I will post a new version of this patch with the max_sectors limit set
to 1 MiB.
Thanks,
Bart.
