On Tue, Dec 29, 2020 at 02:40:57PM +0100, Stefan Lederer wrote: > Hello dear list, > > (I hope I do not annoy you as a simple application programmer) > > for a seminar paper at my university we reproduced the 2009 paper > "Pathologies of big data" by Jacobs, where he basically reads a > 100GB file sequentially from a HDD with some light processing. > > We have a PCIE4.0 SSD with up to 7GB/s reading (Samsung 980) but > nothing we programmed so far comes even close to that speed (regular > read(), mmap() with optional threads, io_uring, multi-process) so we > wonder if it is possible at all? > > According to iostat mmap is the fastest with 4GB/s and a queue depth > of ~3. All other approaches do not go beyond 2.5GB/s. > > Also we get some strange effects like sequential read() with 16KB > buffers being faster than one with 16MB and io_uring being alot > slower than mmap (all tested on Manjaro with kernel 5.8/5.10 and ext4). > > So, now we are quite lost and would appreciate a hint into the right > direction :) > > What is neccesary to simply read 100GB of data at 7GB/s? Is your device running at gen4 speed? Easiest way to tell with an nvme ssd (assuming you're reading from /dev/nvme0n1) is something like: # cat /sys/block/nvme0n1/device/device/current_link_speed If it says less than 16GT/s, then it can't read at 7GB/s.
