On Mon, 22 Jun 2020 at 16:54, David Rowley <dgrowle...@gmail.com> wrote: > I also tested this an AMD machine running Ubuntu 20.04 on kernel > version 5.4.0-37. I used the same 100GB table I mentioned in [1], but > with the query "select * from t where a < 0;", which saves having to > do any aggregate work.
I just wanted to add a note here that Thomas and I just discussed this a bit offline. He recommended I try setting the kernel readhead a bit higher. It was set to 128kB, so I cranked it up to 2MB with: sudo blockdev --setra 4096 /dev/nvme0n1p2 I didn't want to run the full test again as it took quite a long time, so I just tried with 32 workers. The first two results here are taken from the test results I just posted 1 hour ago. Master readhead=128kB = 89921.283 ms v2 patch readhead=128kB = 36085.642 ms master readhead=2MB = 60984.905 ms v2 patch readhead=2MB = 22611.264 ms There must be a fairly large element of reading from the page cache there since 22.6 seconds means 4528MB/sec over the 100GB table. The maximum for a PCIe 3.0 x4 slot is 3940MB/s David
