Vec vs HashMap read performance (ObjectId keys + rustc_hash)

Cargo.toml

[package]
name = "indexed_hash_map"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
mongodb = "2.3.0"
rustc-hash = "1.1.0"
rand = "0.8.5"
criterion = "0.4.0"

[profile.dev]
opt-level = 0

[profile.release]
opt-level = 3

main.rs

use core::cell::RefCell;
use mongodb::bson::oid;
use rand::prelude::*;
use rustc_hash::FxHashMap;
use std::rc::Rc;

// use std::hint::black_box;
use criterion::black_box;
use std::time::Instant;

pub struct Item {
    pub id: oid::ObjectId,
    pub idx: usize,
    pub out_id: Vec<oid::ObjectId>,
    pub out_idx: Vec<usize>,
}

pub struct Edge {
    pub id: oid::ObjectId,
    pub idx: usize,
    pub from_id: oid::ObjectId,
    pub from_idx: usize,
    pub to_id: oid::ObjectId,
    pub to_idx: usize,
}

pub struct ItemRef {
    pub id: oid::ObjectId,
    pub idx: usize,
    pub out: Vec<Rc<RefCell<EdgeRef>>>,
}

pub struct EdgeRef {
    pub id: oid::ObjectId,
    pub from_idx: usize,
    pub from: Rc<RefCell<ItemRef>>,
    pub to_idx: usize,
    pub to: Rc<RefCell<ItemRef>>,
}

const TOTAL: usize = 3000;
const NODE_EDGES: usize = 3;
const READS: usize = 1 + NODE_EDGES * 2;

fn main() {
    hash();
    vec();
    cell();
}

fn hash() {
    let mut item_map = FxHashMap::default(); // HashMap with rustc_hash, should be fast?
    let mut edge_map = FxHashMap::default(); // HashMap with rustc_hash, should be fast?
    let mut item_keys = Vec::new(); // remember used keys
    let mut rng = rand::thread_rng();
    for idx in 0..TOTAL {
        // generate 1K records same way
        let item = Item {
            id: oid::ObjectId::new(),
            idx,
            out_idx: Vec::new(),
            out_id: Vec::new(),
        };
        item_keys.push(item.id); // push rec to map this time
        item_map.insert(item.id, item); // remember key used
    }

    let mut ridx = 0;
    for idx in 0..TOTAL {
        // generate total random edges
        for _ in 0..NODE_EDGES {
            let from_idx = idx;
            let to_idx = rng.gen_range(0..TOTAL);
            let from_id = &item_keys[from_idx];
            let to_id = &item_keys[to_idx];
            let edge = Edge {
                id: oid::ObjectId::new(), // new random UUID
                idx: ridx,                // index
                from_idx: item_map[from_id].idx,
                to_idx: item_map[to_id].idx,
                from_id: item_map[from_id].id,
                to_id: item_map[to_id].id,
            };
            let from = item_map.get_mut(from_id).unwrap();
            from.out_id.push(edge.id);
            from.out_idx.push(edge.idx);
            edge_map.insert(edge.id, edge);
            ridx += 1;
        }
    }

    let now = Instant::now();

    for _ in 0..TOTAL {
        // total runs x total records = 1M reads
        for idx in &item_keys {
            let item = &item_map[&idx];
            for ri in 0..NODE_EDGES {
                black_box(&item_map[&edge_map[&item.out_id[ri]].to_id]); // just to not let compiler skip this code
            }
        }
    }
    let elapsed = now.elapsed();
    println!(
        "{:.2?}M HashMap (rustc_hash) read time: {:.2?}",
        READS * TOTAL * TOTAL / 1000000,
        elapsed
    );
}

fn vec() {
    let mut item_vec = Vec::new(); // items registry
    let mut edge_vec = Vec::new(); // items registry
    let mut item_indexes = Vec::new(); // list of used item indexes (just to iterate over Vec of key when benching same as for HashMap)
    let mut rng = rand::thread_rng();

    for idx in 0..TOTAL {
        // generate total records
        let item = Item {
            id: oid::ObjectId::new(), // new random UUID
            idx,                      // index
            out_id: Vec::new(),
            out_idx: Vec::new(),
        };
        item_vec.push(item); //push to Vec as main storage
        item_indexes.push(idx); // remember indexes used
    }
    let mut ridx = 0;
    for idx in 0..TOTAL {
        // generate total random edges
        for _ in 0..NODE_EDGES {
            let from_idx = idx;
            let to_idx = rng.gen_range(0..TOTAL);
            let edge = Edge {
                id: oid::ObjectId::new(), // new random UUID
                idx: ridx,                // index
                from_idx,
                to_idx,
                from_id: item_vec[from_idx].id,
                to_id: item_vec[to_idx].id,
            };
            item_vec[from_idx].out_id.push(edge.id);
            item_vec[from_idx].out_idx.push(edge.idx);
            edge_vec.push(edge);
            ridx += 1;
        }
    }

    let now = Instant::now();
    item_indexes.shuffle(&mut rng);
    for _ in 0..TOTAL {
        // total runs x total records = 1M reads
        for idx in &item_indexes {
            let item = &item_vec[*idx]; // noob here, not sure if doing nice here with ref/deref
            for ri in 0..NODE_EDGES {
                black_box(&item_vec[edge_vec[item.out_idx[ri]].to_idx]); // just to not let compiler skip this code
            }
        }
    }
    let elapsed = now.elapsed();
    println!(
        "{:.2?}M Vec read time: {:.2?}",
        READS * TOTAL * TOTAL / 1000000,
        elapsed
    );
}

fn cell() {
    let mut item_vec = Vec::new();
    let mut edge_vec = Vec::new(); // HashMap with rustc_hash, should be fast?
    let mut item_indexes = Vec::new(); // remember used keys
    let mut rng = rand::thread_rng();
    for idx in 0..TOTAL {
        // generate 1K records same way
        let item = Rc::new(RefCell::new(ItemRef {
            id: oid::ObjectId::new(),
            idx,
            out: Vec::new(),
        }));
        item_indexes.push(item.borrow().idx); // push rec to map this time
        item_vec.push(item); // remember key used
    }

    for idx in 0..TOTAL {
        // generate total random edges
        for _ in 0..NODE_EDGES {
            let from_idx = idx;
            let to_idx = rng.gen_range(0..TOTAL);
            let edge = Rc::new(RefCell::new(EdgeRef {
                id: oid::ObjectId::new(), // new random UUID
                from_idx,
                to_idx,
                from: item_vec[from_idx].clone(),
                to: item_vec[to_idx].clone(),
            }));
            edge.borrow_mut().from.borrow_mut().out.push(edge.clone());

            edge_vec.push(edge);
        }
    }

    let now = Instant::now();

    for _ in 0..TOTAL {
        // total runs x total records = 1M reads
        for idx in &item_indexes {
            let item = &item_vec[*idx];
            for ri in 0..NODE_EDGES {
                black_box(&item.borrow().out[ri].borrow().to); // just to not let compiler skip this code
            }
        }
    }
    let elapsed = now.elapsed();
    println!(
        "{:.2?}M Vec/Rc/RefCell read time: {:.2?}",
        READS * TOTAL * TOTAL / 1000000,
        elapsed
    );
}

terminal

63M HashMap (rustc_hash) read time: 22,645s
63M Vec read time: 103,53ms
63M Vec/Rc/RefCell read time: 65.88ms

I've uploaded this as example

https://github.com/Mart-Bogdan-TMP/html-parser-benchmark/blob/main/rust-parsers/benches/my_benchmark.rs

But I don't know perhaps it's to complex example. You could try examples from documtation of criterion from link of my original comment

This exact case is ultimately simple and isolated, so my approach is good enough.

That's bold claim.

You can't guarante that it would give same results on each execution.