Increase the IO_THREADS_MAX_NUM. (#1220)

### Description

This patch try to increase the max number of io-threads from 16(128) to
256 for below reasons:

1. The core number increases a lot in the modern server processors, for
example, the [Sierra
Forest](https://en.wikipedia.org/wiki/Sierra_Forest) processors are
targeted towards with up to **288** cores.
Due to limitation of **_io-threads_** number (16 and 128 ), benchmark
like https://openbenchmarking.org/test/pts/valkey even cannot run on a
high core count server.

2. For some workloads, the bottleneck could be main thread, but for the
other workloads, big key/value which caused heavy io, the bottleneck
could be the io-threads, for example benchmark `memtier_benchmark -s
127.0.0.1 -p 9001 "--data-size" "20000" --ratio 1:0 --key-pattern P:P
--key-minimum=1 --key-maximum 1000000 --test-time 180 -c 50 -t 16
--hide-histogram`. The QPS is still scalable after 16 io-threads.

![image](https://github.com/user-attachments/assets/e980f805-a162-44be-b03e-ab37a9c489cf)
**Fig 1. QPS Scale factor with io-threads number grows.**

Signed-off-by: Lipeng Zhu <[email protected]>
Co-authored-by: Wangyang Guo <[email protected]>

src/config.c

@@ -629,9 +629,6 @@ void loadServerConfigFromString(char *config) {
     if (server.config_hz < CONFIG_MIN_HZ) server.config_hz = CONFIG_MIN_HZ;
     if (server.config_hz > CONFIG_MAX_HZ) server.config_hz = CONFIG_MAX_HZ;
 
-    /* To ensure backward compatibility when io_threads_num is according to the previous maximum of 128. */
-    if (server.io_threads_num > IO_THREADS_MAX_NUM) server.io_threads_num = IO_THREADS_MAX_NUM;
-
     sdsfreesplitres(lines, totlines);
     reading_config_file = 0;
     return;
@@ -3202,8 +3199,8 @@ standardConfig static_configs[] = {
 
     /* Integer configs */
     createIntConfig("databases", NULL, IMMUTABLE_CONFIG, 1, INT_MAX, server.dbnum, 16, INTEGER_CONFIG, NULL, NULL),
-    createIntConfig("port", NULL, MODIFIABLE_CONFIG, 0, 65535, server.port, 6379, INTEGER_CONFIG, NULL, updatePort),                    /* TCP port. */
-    createIntConfig("io-threads", NULL, DEBUG_CONFIG | IMMUTABLE_CONFIG, 1, 128, server.io_threads_num, 1, INTEGER_CONFIG, NULL, NULL), /* Single threaded by default */
+    createIntConfig("port", NULL, MODIFIABLE_CONFIG, 0, 65535, server.port, 6379, INTEGER_CONFIG, NULL, updatePort),                                   /* TCP port. */
+    createIntConfig("io-threads", NULL, DEBUG_CONFIG | IMMUTABLE_CONFIG, 1, IO_THREADS_MAX_NUM, server.io_threads_num, 1, INTEGER_CONFIG, NULL, NULL), /* Single threaded by default */
     createIntConfig("events-per-io-thread", NULL, MODIFIABLE_CONFIG, 0, INT_MAX, server.events_per_io_thread, 2, INTEGER_CONFIG, NULL, NULL),
     createIntConfig("prefetch-batch-max-size", NULL, MODIFIABLE_CONFIG, 0, 128, server.prefetch_batch_max_size, 16, INTEGER_CONFIG, NULL, NULL),
     createIntConfig("auto-aof-rewrite-percentage", NULL, MODIFIABLE_CONFIG, 0, INT_MAX, server.aof_rewrite_perc, 100, INTEGER_CONFIG, NULL, NULL),

src/config.h

@@ -338,7 +338,7 @@ void setcpuaffinity(const char *cpulist);
 #define HAVE_FADVISE
 #endif
 
-#define IO_THREADS_MAX_NUM 16
+#define IO_THREADS_MAX_NUM 256
 
 #ifndef CACHE_LINE_SIZE
 #if defined(__aarch64__) && defined(__APPLE__)