Messages in this thread | | | Subject | Re: Why is MEGASAS_SAS_QD set to 256? | From | Yu Kuai <> | Date | Mon, 28 Nov 2022 11:09:28 +0800 |
| |
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 > > . >
| |