Add an environment variable for controlling the default Thread quantum

This commit adds an environment variable `RUBY_THREAD_TIMESLICE` for
specifying the default thread quantum in milliseconds.  You can adjust
this variable to tune throughput, which is especially useful on
multithreaded systems that are mixing CPU bound work and IO bound work.

The default quantum remains 100ms.

[Feature #20861]

Co-Authored-By: John Hawthorn <[email protected]>

4 files changed, 13 insertions, 12 deletions

diff --git a/man/ruby.1 b/man/ruby.1
index 905c712f4c..b1b8e42401 100644
--- a/man/ruby.1
+++ b/man/ruby.1
@@ -537,6 +537,10 @@ Introduced in Ruby 3.3, default: unset.
 .Pp
 .It Ev RUBY_PAGER
 The pager command that will be used for
+.Pp
+.It Ev RUBY_THREAD_TIMESLICE
+Sets the default thread time slice (thread quantum) in milliseconds.
+Introduced in Ruby 3.4, default: 100ms.
 .Fl -help
 option.
 Introduced in Ruby 3.0, default:
diff --git a/thread.c b/thread.c
index 86c7392e9c..2d2951d18a 100644
--- a/thread.c
+++ b/thread.c
@@ -116,6 +116,8 @@ static VALUE sym_immediate;
 static VALUE sym_on_blocking;
 static VALUE sym_never;
 
+static uint32_t thread_default_quantum_ms = 100;
+
 #define THREAD_LOCAL_STORAGE_INITIALISED FL_USER13
 #define THREAD_LOCAL_STORAGE_INITIALISED_P(th) RB_FL_TEST_RAW((th), THREAD_LOCAL_STORAGE_INITIALISED)
 
@@ -2538,7 +2540,7 @@ rb_threadptr_execute_interrupts(rb_thread_t *th, int blocking_timing)
         }
 
         if (timer_interrupt) {
-            uint32_t limits_us = TIME_QUANTUM_USEC;
+            uint32_t limits_us = thread_default_quantum_ms * 1000;
 
             if (th->priority > 0)
                 limits_us <<= th->priority;
@@ -5506,6 +5508,12 @@ Init_Thread(void)
     rb_define_method(cThGroup, "enclosed?", thgroup_enclosed_p, 0);
     rb_define_method(cThGroup, "add", thgroup_add, 1);
 
+    const char * ptr = getenv("RUBY_THREAD_TIMESLICE");
+
+    if (ptr) {
+        thread_default_quantum_ms = (uint32_t)strtol(ptr, NULL, 0);
+    }
+
     {
         th->thgroup = th->ractor->thgroup_default = rb_obj_alloc(cThGroup);
         rb_define_const(cThGroup, "Default", th->thgroup);
diff --git a/thread_none.c b/thread_none.c
index 1582609821..8d32aaabee 100644
--- a/thread_none.c
+++ b/thread_none.c
@@ -19,10 +19,6 @@
 # include "wasm/machine.h"
 #endif
 
-#define TIME_QUANTUM_MSEC (100)
-#define TIME_QUANTUM_USEC (TIME_QUANTUM_MSEC * 1000)
-#define TIME_QUANTUM_NSEC (TIME_QUANTUM_USEC * 1000)
-
 // Do nothing for GVL
 static void
 thread_sched_to_running(struct rb_thread_sched *sched, rb_thread_t *th)
diff --git a/thread_pthread.c b/thread_pthread.c
index 09c2969e8c..357454f9b5 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -317,13 +317,6 @@ static void threadptr_trap_interrupt(rb_thread_t *);
 #define native_thread_yield() ((void)0)
 #endif
 
-/* 100ms.  10ms is too small for user level thread scheduling
- * on recent Linux (tested on 2.6.35)
- */
-#define TIME_QUANTUM_MSEC (100)
-#define TIME_QUANTUM_USEC (TIME_QUANTUM_MSEC * 1000)
-#define TIME_QUANTUM_NSEC (TIME_QUANTUM_USEC * 1000)
-
 static void native_thread_dedicated_inc(rb_vm_t *vm, rb_ractor_t *cr, struct rb_native_thread *nt);
 static void native_thread_dedicated_dec(rb_vm_t *vm, rb_ractor_t *cr, struct rb_native_thread *nt);
 static void native_thread_assign(struct rb_native_thread *nt, rb_thread_t *th);