Refactor scheduler and switch to a spinner thread concept for wakeups

This also adds a counter for idle/sleeping threads to avoid checking every thread when everyone is running.

Author: gbaraldi

base/Base.jl

@@ -137,7 +137,7 @@ const liblapack_name = libblas_name
 # Note that `atomics.jl` here should be deprecated
 Core.eval(Threads, :(include("atomics.jl")))
 include("channels.jl")
-include("partr.jl")
+include("scheduler/scheduler.jl")
 include("task.jl")
 include("threads_overloads.jl")
 include("weakkeydict.jl")

base/partr.jl renamed to base/scheduler/partr.jl

@@ -19,63 +19,6 @@ const heap_d = UInt32(8)
 const heaps = [Vector{taskheap}(undef, 0), Vector{taskheap}(undef, 0)]
 const heaps_lock = [SpinLock(), SpinLock()]
 
-
-"""
-    cong(max::UInt32)
-
-Return a random UInt32 in the range `1:max` except if max is 0, in that case return 0.
-"""
-cong(max::UInt32) = iszero(max) ? UInt32(0) : rand_ptls(max) + UInt32(1) #TODO: make sure users don't use 0 and remove this check
-
-get_ptls_rng() = ccall(:jl_get_ptls_rng, UInt64, ())
-
-set_ptls_rng(seed::UInt64) = ccall(:jl_set_ptls_rng, Cvoid, (UInt64,), seed)
-
-"""
-    rand_ptls(max::UInt32)
-
-Return a random UInt32 in the range `0:max-1` using the thread-local RNG
-state. Max must be greater than 0.
-"""
-Base.@assume_effects :removable :inaccessiblememonly :notaskstate function rand_ptls(max::UInt32)
-    rngseed = get_ptls_rng()
-    val, seed = rand_uniform_max_int32(max, rngseed)
-    set_ptls_rng(seed)
-    return val % UInt32
-end
-
-# This implementation is based on OpenSSLs implementation of rand_uniform
-# https://github.com/openssl/openssl/blob/1d2cbd9b5a126189d5e9bc78a3bdb9709427d02b/crypto/rand/rand_uniform.c#L13-L99
-# Comments are vendored from their implementation as well.
-# For the original developer check the PR to swift https://github.com/apple/swift/pull/39143.
-
-# Essentially it boils down to incrementally generating a fixed point
-# number on the interval [0, 1) and multiplying this number by the upper
-# range limit.  Once it is certain what the fractional part contributes to
-# the integral part of the product, the algorithm has produced a definitive
-# result.
-"""
-    rand_uniform_max_int32(max::UInt32, seed::UInt64)
-
-Return a random UInt32 in the range `0:max-1` using the given seed.
-Max must be greater than 0.
-"""
-Base.@assume_effects :total function rand_uniform_max_int32(max::UInt32, seed::UInt64)
-    if max == UInt32(1)
-        return UInt32(0), seed
-    end
-    # We are generating a fixed point number on the interval [0, 1).
-    # Multiplying this by the range gives us a number on [0, upper).
-    # The high word of the multiplication result represents the integral part
-    # This is not completely unbiased as it's missing the fractional part of the original implementation but it's good enough for our purposes
-    seed = UInt64(69069) * seed + UInt64(362437)
-    prod = (UInt64(max)) * (seed % UInt32) # 64 bit product
-    i = prod >> 32 % UInt32 # integral part
-    return i % UInt32, seed
-end
-
-
-
 function multiq_sift_up(heap::taskheap, idx::Int32)
     while idx > Int32(1)
         parent = (idx - Int32(2)) ÷ heap_d + Int32(1)
@@ -147,10 +90,10 @@ function multiq_insert(task::Task, priority::UInt16)
 
     task.priority = priority
 
-    rn = cong(heap_p)
+    rn = Base.Scheduler.cong(heap_p)
     tpheaps = heaps[tp]
     while !trylock(tpheaps[rn].lock)
-        rn = cong(heap_p)
+        rn = Base.Scheduler.cong(heap_p)
     end
 
     heap = tpheaps[rn]
@@ -190,8 +133,8 @@ function multiq_deletemin()
         if i == heap_p
             return nothing
         end
-        rn1 = cong(heap_p)
-        rn2 = cong(heap_p)
+        rn1 = Base.Scheduler.cong(heap_p)
+        rn2 = Base.Scheduler.cong(heap_p)
         prio1 = tpheaps[rn1].priority
         prio2 = tpheaps[rn2].priority
         if prio1 > prio2
@@ -211,7 +154,21 @@ function multiq_deletemin()
     heap = tpheaps[rn1]
     task = heap.tasks[1]
     if ccall(:jl_set_task_tid, Cint, (Any, Cint), task, tid-1) == 0
+        # This task is stuck to a thread that's likely sleeping, move the task to it's private queue and wake it up
+        # We move this out of the queue to avoid spinning on it
+        ntasks = heap.ntasks
+        @atomic :monotonic heap.ntasks = ntasks - Int32(1)
+        heap.tasks[1] = heap.tasks[ntasks]
+        Base._unsetindex!(heap.tasks, Int(ntasks))
+        prio1 = typemax(UInt16)
+        if ntasks > 1
+            multiq_sift_down(heap, Int32(1))
+            prio1 = heap.tasks[1].priority
+        end
+        @atomic :monotonic heap.priority = prio1
+        push!(workqueue_for(tid), t)
         unlock(heap.lock)
+        ccall(:jl_wakeup_thread, Cvoid, (Int16,), (tid - 1) % Int16)
         @goto retry
     end
     ntasks = heap.ntasks
@@ -243,4 +200,9 @@ function multiq_check_empty()
     return true
 end
 
+
+enqueue!(t::Task) = multiq_insert(t, t.priority)
+dequeue!() = multiq_deletemin()
+checktaskempty() = multiq_check_empty()
+
 end

base/scheduler/scheduler.jl

@@ -0,0 +1,74 @@
+# This file is a part of Julia. License is MIT: https://julialang.org/license
+
+module Scheduler
+
+"""
+    cong(max::UInt32)
+
+Return a random UInt32 in the range `1:max` except if max is 0, in that case return 0.
+"""
+cong(max::UInt32) = iszero(max) ? UInt32(0) : rand_ptls(max) + UInt32(1) #TODO: make sure users don't use 0 and remove this check
+
+get_ptls_rng() = ccall(:jl_get_ptls_rng, UInt64, ())
+
+set_ptls_rng(seed::UInt64) = ccall(:jl_set_ptls_rng, Cvoid, (UInt64,), seed)
+
+"""
+    rand_ptls(max::UInt32)
+
+Return a random UInt32 in the range `0:max-1` using the thread-local RNG
+state. Max must be greater than 0.
+"""
+Base.@assume_effects :removable :inaccessiblememonly :notaskstate function rand_ptls(max::UInt32)
+    rngseed = get_ptls_rng()
+    val, seed = rand_uniform_max_int32(max, rngseed)
+    set_ptls_rng(seed)
+    return val % UInt32
+end
+
+# This implementation is based on OpenSSLs implementation of rand_uniform
+# https://github.com/openssl/openssl/blob/1d2cbd9b5a126189d5e9bc78a3bdb9709427d02b/crypto/rand/rand_uniform.c#L13-L99
+# Comments are vendored from their implementation as well.
+# For the original developer check the PR to swift https://github.com/apple/swift/pull/39143.
+
+# Essentially it boils down to incrementally generating a fixed point
+# number on the interval [0, 1) and multiplying this number by the upper
+# range limit.  Once it is certain what the fractional part contributes to
+# the integral part of the product, the algorithm has produced a definitive
+# result.
+"""
+    rand_uniform_max_int32(max::UInt32, seed::UInt64)
+
+Return a random UInt32 in the range `0:max-1` using the given seed.
+Max must be greater than 0.
+"""
+Base.@assume_effects :total function rand_uniform_max_int32(max::UInt32, seed::UInt64)
+    if max == UInt32(1)
+        return UInt32(0), seed
+    end
+    # We are generating a fixed point number on the interval [0, 1).
+    # Multiplying this by the range gives us a number on [0, upper).
+    # The high word of the multiplication result represents the integral part
+    # This is not completely unbiased as it's missing the fractional part of the original implementation but it's good enough for our purposes
+    seed = UInt64(69069) * seed + UInt64(362437)
+    prod = (UInt64(max)) * (seed % UInt32) # 64 bit product
+    i = prod >> 32 % UInt32 # integral part
+    return i % UInt32, seed
+end
+
+include("scheduler/partr.jl")
+
+const ChosenScheduler = Partr
+
+
+
+# Scheduler interface:
+    # enqueue! which pushes a runnable Task into it
+    # dequeue! which pops a runnable Task from it
+    # checktaskempty which returns true if the scheduler has no available Tasks
+
+enqueue!(t::Task) = ChosenScheduler.enqueue!(t)
+dequeue!() = ChosenScheduler.dequeue!()
+checktaskempty() = ChosenScheduler.checktaskempty()
+
+end

base/task.jl

@@ -937,7 +937,6 @@ end
 
 function enq_work(t::Task)
     (t._state === task_state_runnable && t.queue === nothing) || error("schedule: Task not runnable")
-
     # Sticky tasks go into their thread's work queue.
     if t.sticky
         tid = Threads.threadid(t)
@@ -968,19 +967,44 @@ function enq_work(t::Task)
             ccall(:jl_set_task_tid, Cint, (Any, Cint), t, tid-1)
             push!(workqueue_for(tid), t)
         else
-            # Otherwise, put the task in the multiqueue.
-            Partr.multiq_insert(t, t.priority)
+            # Otherwise, push the task to the scheduler
+            Scheduler.enqueue!(t)
             tid = 0
         end
     end
-    ccall(:jl_wakeup_thread, Cvoid, (Int16,), (tid - 1) % Int16)
+
+    if (tid == 0)
+        Core.Intrinsics.atomic_fence(:sequentially_consistent)
+        n_spinning = Core.Intrinsics.atomic_pointerref(cglobal(:jl_n_threads_spinning, Cint), :monotonic)
+        n_spinning == 0 && ccall(:jl_add_spinner, Cvoid, ())
+    else
+        ccall(:jl_wakeup_thread, Cvoid, (Int16,), (tid - 1) % Int16)
+    end
+    # n_spinning = Core.Intrinsics.atomic_pointerref(cglobal(:jl_n_threads, Cint), :acquire)
+    # n_spinning == 0 && ccall(:jl_add_spinner, Cvoid, ())
     return t
 end
 
+const ChildFirst = false
+
 function schedule(t::Task)
     # [task] created -scheduled-> wait_time
     maybe_record_enqueued!(t)
-    enq_work(t)
+    if ChildFirst
+        ct = current_task()
+        if ct.sticky || t.sticky
+            maybe_record_enqueued!(t)
+            enq_work(t)
+        else
+            maybe_record_enqueued!(t)
+            enq_work(ct)
+            yieldto(t)
+        end
+    else
+        maybe_record_enqueued!(t)
+        enq_work(t)
+    end
+    return t
 end
 
 """
@@ -1176,10 +1200,10 @@ function trypoptask(W::StickyWorkqueue)
         end
         return t
     end
-    return Partr.multiq_deletemin()
+    return Scheduler.dequeue!()
 end
 
-checktaskempty = Partr.multiq_check_empty
+checktaskempty = Scheduler.checktaskempty
 
 @noinline function poptask(W::StickyWorkqueue)
     task = trypoptask(W)

src/jl_exported_data.inc

@@ -157,6 +157,7 @@
 #define JL_EXPORTED_DATA_SYMBOLS(XX) \
     XX(jl_n_threadpools, int) \
     XX(jl_n_threads, _Atomic(int)) \
+    XX(jl_n_threads_spinning, _Atomic(int)) \
     XX(jl_n_gcthreads, int) \
     XX(jl_options, jl_options_t) \
     XX(jl_task_gcstack_offset, int) \

src/jl_exported_funcs.inc

@@ -449,6 +449,7 @@
     XX(jl_tagged_gensym) \
     XX(jl_take_buffer) \
     XX(jl_task_get_next) \
+    XX(jl_add_spinner) \
     XX(jl_task_stack_buffer) \
     XX(jl_termios_size) \
     XX(jl_test_cpu_feature) \

src/julia.h

@@ -2055,6 +2055,7 @@ JL_DLLEXPORT jl_sym_t *jl_get_ARCH(void) JL_NOTSAFEPOINT;
 JL_DLLIMPORT jl_value_t *jl_get_libllvm(void) JL_NOTSAFEPOINT;
 extern JL_DLLIMPORT int jl_n_threadpools;
 extern JL_DLLIMPORT _Atomic(int) jl_n_threads;
+extern JL_DLLIMPORT _Atomic(int) jl_n_threads_spinning; // Scheduler internal counter
 extern JL_DLLIMPORT int jl_n_gcthreads;
 extern int jl_n_markthreads;
 extern int jl_n_sweepthreads;

src/julia_threads.h

@@ -209,6 +209,7 @@ typedef struct _jl_tls_states_t {
         uint64_t uv_run_leave;
         uint64_t sleep_enter;
         uint64_t sleep_leave;
+        uint64_t woken_up;
     )
 
     // some hidden state (usually just because we don't have the type's size declaration)