Hi,
在 2022/11/27 17:42, Ming Lei 写道:
On Sat, Nov 26, 2022 at 02:08:02PM +0800, Yu Kuai wrote:
Hi, Ming
在 2022/11/26 10:18, Ming Lei 写道:
If you want aggressive merge on sequential IO workload, the queue depth need
to be a bit less, then more requests can be staggered into scheduler queue,
and merge chance is increased.
But if nr_requests >= queue_depth, it seems to me elevator will have no
effect, no request can be merged or sorted by scheduler, right?
Yeah.
If nr_requests <= queue_depth, every request can be queued to
driver/device, so requests won't be merged by scheduler.
But plug merge still works if IOs are submitted as batch.
Yes, io can still be merged by plug. I just find it a little werid to
set default elevator as deadline, and default queue_depth to 256. Which
means deadline here is useless.
If you want good perf on random IO perf, the queue depth needs to
be deep enough to have enough parallelism for saturating SSD internal.
But we don't recognize sequential/random IO pattern, and usually fixed
queue depth is used.
Is it possible to use none elevator and set large queue_depth if nvme is
used in this case?
Yeah, if the storage is SSD, usually none with bigger queue_depth should
help, and usually 256 should be enough to saturate one single SSD for
one well implemented driver.
Yes, I'm testing with multiple SSDs / NVMEs, and I can't get optimal
performance by default.
Thanks,
Kuai
Thanks
Ming
.
