On Wed, Nov 2, 2022 at 12:42:11AM +1300, Thomas Munro wrote: > On Wed, Nov 2, 2022 at 12:09 AM Andy Fan <zhihui.fan1...@gmail.com> wrote: > > By theory, Why does the preferch make thing better? I am asking this > > because I think we need to read the data from buffer to cache line once > > in either case (I'm obvious wrong in face of the test result.) > > CPUs have several different kinds of 'hardware prefetchers' (worth > reading about), that look out for sequential and striding patterns and > try to get the cache line ready before you access it. Using the > prefetch instructions explicitly is called 'software prefetching' > (special instructions inserted by programmers or compilers). The > theory here would have to be that the hardware prefetchers couldn't > pick up the pattern, but we know how to do it. The exact details of > the hardware prefetchers vary between chips, and there are even some > parameters you can adjust in BIOS settings. One idea is that the > hardware prefetchers are generally biased towards increasing > addresses, but our tuples tend to go backwards on the page[1]. It's > possible that some other CPUs can detect backwards strides better, but > since real world tuples aren't of equal size anyway, there isn't > really a fixed stride at all, so software prefetching seems quite > promising for this... > > [1] > https://www.postgresql.org/docs/current/storage-page-layout.html#STORAGE-PAGE-LAYOUT-FIGURE
I remember someone showing that having our item pointers at the _end_ of the page and tuples at the start moving toward the end increased performance significantly. -- Bruce Momjian <br...@momjian.us> https://momjian.us EDB https://enterprisedb.com Indecision is a decision. Inaction is an action. Mark Batterson
