examples: add ycsb-d simulation example

examples/ycsbd/README.md

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+This example shows how to run YCSB-like benchmarks in kvbench by utilizing its customizable
+workloads. Here we simulate the YCSB-D workload where there are 5% inserts/writes and 95% reads.
+The key distribution still follows Zipfian but the hotspot is always the latest written key.
+
+Unlike YCSB that really inserts a key, kvbench assumes a fixed-sized key space, and a write
+operation can be either an insert or an update internally. For the latest key, it should not make
+a huge difference as it's usually cached.
+
+The workload file, `ycsbd.toml` is as follows:
+
+```toml
+[global]
+threads = 1
+repeat = 1
+klen = 8
+vlen = 16
+kmin = 0
+kmax = 1000000
+
+[[benchmark]]
+set_perc = 100
+get_perc = 0
+del_perc = 0
+repeat = 1
+dist = "shufflep"
+report = "hidden"
+
+[[benchmark]]
+timeout = 3
+set_perc = 5
+get_perc = 95
+del_perc = 0
+dist = "latest"
+report = "finish"
+```
+
+In the first phase, all worker threads fill the key space of the store, and the metrics are hidden.
+Note that the loading is done using `shufflep` instead of `incrementp`. With `shufflep`, the order
+of the keys in the key space is shuffled and each key is written exactly once.
+In the second phase, worker threads execute the workload using the built-in `latest` key
+distribution.
+
+The script file `run.sh` runs this benchmark against multiple stores with different number of
+threads. The number of threads are dynamically adjusted via `global.threads` environment variable.
+
+AMD Ryzen 9 5950X CPU 0-15 results ([pdf](ycsbd.pdf)):
+
+![ycsbd](ycsbd.png)

examples/ycsbd/plot.gpl

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+set terminal pdf
+set ylabel "Throughput (MOP/s)"
+set xlabel "Threads"
+set key left top
+
+set output "ycsbd.pdf"
+plot [0:17] [0:] \
+    "chashmap.txt" using 2:14 with lp ti "chashmap",\
+    "contrie.txt" using 2:14 with lp ti "contrie",\
+    "dashmap.txt" using 2:14 with lp ti "dashmap",\
+    "flurry.txt" using 2:14 with lp ti "flurry",\
+    "papaya.txt" using 2:14 with lp ti "papaya",\
+    "scchashmap.txt" using 2:14 with lp ti "scchashmap",\
+    "mutex_hashmap.txt" using 2:14 with lp ti "mutexhashmap",\
+    "rwlock_hashmap.txt" using 2:14 with lp ti "rwlockhashmap"

examples/ycsbd/run.sh

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+#!/bin/bash
+
+DIR="$( cd -- "$(dirname "$0")" >/dev/null 2>&1 ; pwd -P )"
+
+cargo +stable build --profile release-lto --all-features
+
+STORE_DIR=$DIR/../../presets/stores
+BENCHMARK=$DIR/ycsbd.toml
+
+STORES="chashmap contrie dashmap flurry papaya scchashmap mutex_hashmap rwlock_hashmap"
+
+for s in $STORES; do
+    echo $s
+    rm $s.txt 2>/dev/null
+    for t in `seq 1 16`; do
+        data="$(env global.threads=$t cargo +stable run --profile release-lto --all-features -- bench -s $STORE_DIR/$s.toml -b $BENCHMARK 2>/dev/null)"
+        echo "threads $t $data" | tee -a $s.txt
+    done
+done
+
+gnuplot $DIR/plot.gpl

examples/ycsbd/ycsbd.toml

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+[global]
+threads = 1
+repeat = 1
+klen = 8
+vlen = 16
+kmin = 0
+kmax = 1000000
+
+[[benchmark]]
+set_perc = 100
+get_perc = 0
+del_perc = 0
+repeat = 1
+dist = "shufflep"
+report = "hidden"
+
+[[benchmark]]
+timeout = 3
+set_perc = 5
+get_perc = 95
+del_perc = 0
+dist = "latest"
+report = "finish"