On 8/2/22 1:36 PM, Keith Busch wrote: > device undergoes various represenations for every IO. Each consumes > memory and CPU cycles. When the backing storage is NVMe, the sequence > looks something like the following: > > __user void * > struct iov_iter > struct pages[] > struct bio_vec[] > struct scatterlist[] > __le64[] > > Applications will often use the same buffer for many IO, though, so > these potentially costly per-IO transformations to reach the exact same > hardware descriptor can be skipped. > > The io_uring interface already provides a way for users to register > buffers to get to the 'struct bio_vec[]'. That still leaves the > scatterlist needed for the repeated dma_map_sg(), then transform to > nvme's PRP list format. > > This series takes the registered buffers a step further. A block driver > can implement a new .dma_map() callback to complete the representation > to the hardware's DMA mapped address, and return a cookie so a user can > reference it later for any given IO. When used, the block stack can skip > significant amounts of code, improving CPU utilization, and, if not > bandwidth limited, IOPs. > > The implementation is currently limited to mapping a registered buffer > to a single file. I ran this on my test box to see how we'd do. First the bad news: smaller block size IO seems slower. I ran with QD=8 and used 24 drives, and using t/io_uring (with registered buffers, polled, etc) and a 512b block size I get: IOPS=44.36M, BW=21.66GiB/s, IOS/call=1/1 IOPS=44.64M, BW=21.80GiB/s, IOS/call=2/2 IOPS=44.69M, BW=21.82GiB/s, IOS/call=1/1 IOPS=44.55M, BW=21.75GiB/s, IOS/call=2/2 IOPS=44.93M, BW=21.94GiB/s, IOS/call=1/1 IOPS=44.79M, BW=21.87GiB/s, IOS/call=1/2 and adding -D1 I get: IOPS=43.74M, BW=21.36GiB/s, IOS/call=1/1 IOPS=44.04M, BW=21.50GiB/s, IOS/call=1/1 IOPS=43.63M, BW=21.30GiB/s, IOS/call=2/2 IOPS=43.67M, BW=21.32GiB/s, IOS/call=1/1 IOPS=43.57M, BW=21.28GiB/s, IOS/call=1/2 IOPS=43.53M, BW=21.25GiB/s, IOS/call=2/1 which does regress that workload. Since we avoid more expensive setup at higher block sizes, I tested that too. Here's using 128k IOs with -D0: OPS=972.18K, BW=121.52GiB/s, IOS/call=31/31 IOPS=988.79K, BW=123.60GiB/s, IOS/call=31/31 IOPS=990.40K, BW=123.80GiB/s, IOS/call=31/31 IOPS=987.80K, BW=123.48GiB/s, IOS/call=31/31 IOPS=988.12K, BW=123.52GiB/s, IOS/call=31/31 and here with -D1: IOPS=978.36K, BW=122.30GiB/s, IOS/call=31/31 IOPS=996.75K, BW=124.59GiB/s, IOS/call=31/31 IOPS=996.55K, BW=124.57GiB/s, IOS/call=31/31 IOPS=996.52K, BW=124.56GiB/s, IOS/call=31/31 IOPS=996.54K, BW=124.57GiB/s, IOS/call=31/31 IOPS=996.51K, BW=124.56GiB/s, IOS/call=31/31 which is a notable improvement. Then I checked CPU utilization, switching to IRQ driven IO instead. And the good news there for bs=128K we end up using half the CPU to achieve better performance. So definite win there! Just a quick dump on some quick result, I didn't look further into this just yet. -- Jens Axboe
