| 21 Oct 2025 |
 Eelco | it's funny though that the optimization that skipped parsing 15 GB of NARs (https://github.com/DeterminateSystems/nix-src/pull/238/commits/1f8d587a0df8f9de366640831dade43d17021c30) had basically no observable effect | 21:01:55 |
| Eelco | it's completely dwarfed by the memory allocation / page fault overhead | 21:02:07 |
 Sergei Zimmerman (xokdvium) | Yeah that will do it certainly. Also having a too large buffer on the stack is bad:
https://github.com/NixOS/nix/pull/13877 | 21:03:27 |
| Sergei Zimmerman (xokdvium) | The stack pointer does get decremented one page at a time. Is that the default behavior or some hardening flag? | 21:04:10 |
| Eelco | surprising since stack pages should stay around once paged in | 21:07:22 |
| Eelco | though there is some overhead to handle guard pages | 21:07:42 |
| Eelco | so it has to touch at least 1 byte every 4096 bytes | 21:07:52 |
| Sergei Zimmerman (xokdvium) | Yeah that was the overhead. A loop over all the pages | 21:08:28 |
| Sergei Zimmerman (xokdvium) | 1.23 │ lea -0x10000(%rsp),%r11
0.23 │ 15: sub $0x1000,%rsp
1.01 │ orq $0x0,(%rsp)
59.12 │ cmp %r11,%rsp
0.27 │ ↑ jne 15
| 21:08:42 |
| Eelco | right, that's to avoid a segfault if you have guard pages enabled (which I think is the default) | 21:09:13 |
| Eelco | I would expect the overhead for that loop to be pretty trivial though | 21:09:31 |
| Eelco | in the case where the pages are present | 21:09:52 |
| 22 Oct 2025 |
|  0xcafca changed their profile picture. | 10:21:53 |
