Hi,
在 2025/07/31 17:25, Ming Lei 写道:
On Thu, Jul 31, 2025 at 04:42:10PM +0800, Yu Kuai wrote:
Hi,
在 2025/07/31 16:18, Ming Lei 写道:
batch dispatch may hurt io merge performance which is important for
elevator, so please provide test data on real HDD. & SSD., instead of
null_blk only, and it can be perfect if merge sensitive workload
is evaluated.
Ok, I'll provide test data on HDD and SSD that I have for now.
For the elevator IO merge case, what I have in mind is that we issue
small sequential IO one by one with multiple contexts, so that bios
won't be merged in plug, and this will require IO issue > IO done, is
this case enough?
Long time ago, I investigated one such issue which is triggered in qemu
workload, but not sure if I can find it now.
Also many scsi devices may easily run into queue busy, then scheduler merge
starts to work, and it may perform worse if you dispatch more in this
situation.
I think we won't dispatch more in this case, on the one hand we will get
budgets first, to make sure never dispatch more than queue_depth; on the
onther hand, in the case hctx->dispatch_busy is set, we still fallback
to the old case to dispatch one at a time;
So if the IO merge start because many rqs are accumulated inside
elevator and the driver if full, I will expect IO merge will behave the
same with this set. I'll have a test to check.
Thanks,
Kuai
Thanks,
Ming
.
