Backport concurrent page sweeping

NEWS.md

@@ -38,6 +38,7 @@ Compiler/Runtime improvements
 * All uses of the `@pure` macro in `Base` have been replaced with the now-preferred `Base.@assume_effects` ([#44776]).
 * `invoke(f, invokesig, args...)` calls to a less-specific method than would normally be chosen
   for `f(args...)` are no longer spuriously invalidated when loading package precompile files ([#46010]).
+* The mark phase of the Garbage Collector is now multi-threaded ([#48600]).
 
 Command-line option changes
 ---------------------------
@@ -49,6 +50,8 @@ Command-line option changes
   number of interactive threads to create (`auto` currently means 1) ([#42302]).
 * New option `--heap-size-hint=<size>` suggests a size limit to invoke garbage collection more eagerly.
   The size may be specified in bytes, kilobytes (1000k), megabytes (300M), or gigabytes (1.5G) ([#45369]).
+* New option `--gcthreads` to set how many threads will be used by the Garbage Collector ([#48600]).
+  The default is set to `N/2` where `N` is the amount of worker threads (`--threads`) used by Julia.
 
 Multi-threading changes
 -----------------------

base/options.jl

@@ -11,6 +11,8 @@ struct JLOptions
     cpu_target::Ptr{UInt8}
     nthreadpools::Int16
     nthreads::Int16
+    nmarkthreads::Int16
+    nsweepthreads::Int8
     nthreads_per_pool::Ptr{Int16}
     nprocs::Int32
     machine_file::Ptr{UInt8}

base/threadingconstructs.jl

@@ -130,6 +130,14 @@ function threadpooltids(pool::Symbol)
     end
 end
 
+"""
+    Threads.ngcthreads() -> Int
+
+Returns the number of GC threads currently configured.
+This includes both mark threads and concurrent sweep threads.
+"""
+ngcthreads() = Int(unsafe_load(cglobal(:jl_n_gcthreads, Cint))) + 1
+
 function threading_run(fun, static)
     ccall(:jl_enter_threaded_region, Cvoid, ())
     n = threadpoolsize()

doc/man/julia.1

@@ -118,6 +118,11 @@ supported (Linux and Windows). If this is not supported (macOS) or
 process affinity is not configured, it uses the number of CPU
 threads.
 
+.TP
+--gcthreads <n>
+Enable n GC threads; If unspecified is set to half of the
+compute worker threads.
+
 .TP
 -p, --procs {N|auto}
 Integer value N launches N additional local worker processes `auto` launches as many workers

doc/src/base/multi-threading.md

@@ -10,6 +10,7 @@ Base.Threads.nthreads
 Base.Threads.threadpool
 Base.Threads.nthreadpools
 Base.Threads.threadpoolsize
+Base.Threads.ngcthreads
 ```
 
 See also [Multi-Threading](@ref man-multithreading).

doc/src/manual/command-line-interface.md

@@ -106,8 +106,9 @@ The following is a complete list of command-line switches available when launchi
 |`-e`, `--eval <expr>`                  |Evaluate `<expr>`|
 |`-E`, `--print <expr>`                 |Evaluate `<expr>` and display the result|
 |`-L`, `--load <file>`                  |Load `<file>` immediately on all processors|
-|`-t`, `--threads {N\|auto`}            |Enable N threads; `auto` tries to infer a useful default number of threads to use but the exact behavior might change in the future.  Currently, `auto` uses the number of CPUs assigned to this julia process based on the OS-specific affinity assignment interface, if supported (Linux and Windows). If this is not supported (macOS) or process affinity is not configured, it uses the number of CPU threads.|
-|`-p`, `--procs {N\|auto`}              |Integer value N launches N additional local worker processes; `auto` launches as many workers as the number of local CPU threads (logical cores)|
+|`-t`, `--threads {N\|auto}`            |Enable N threads; `auto` tries to infer a useful default number of threads to use but the exact behavior might change in the future.  Currently, `auto` uses the number of CPUs assigned to this julia process based on the OS-specific affinity assignment interface, if supported (Linux and Windows). If this is not supported (macOS) or process affinity is not configured, it uses the number of CPU threads.|
+| `--gcthreads {N}`                     |Enable N GC threads; If unspecified is set to half of the compute worker threads.|
+|`-p`, `--procs {N\|auto}`              |Integer value N launches N additional local worker processes; `auto` launches as many workers as the number of local CPU threads (logical cores)|
 |`--machine-file <file>`                |Run processes on hosts listed in `<file>`|
 |`-i`                                   |Interactive mode; REPL runs and `isinteractive()` is true|
 |`-q`, `--quiet`                        |Quiet startup: no banner, suppress REPL warnings|

doc/src/manual/environment-variables.md

@@ -315,6 +315,14 @@ then spinning threads never sleep. Otherwise, `$JULIA_THREAD_SLEEP_THRESHOLD` is
 interpreted as an unsigned 64-bit integer (`uint64_t`) and gives, in
 nanoseconds, the amount of time after which spinning threads should sleep.
 
+### [`JULIA_NUM_GC_THREADS`](@id env-gc-threads)
+
+Sets the number of threads used by Garbage Collection. If unspecified is set to
+half of the number of worker threads.
+
+!!! compat "Julia 1.10"
+    The environment variable was added in 1.10
+
 ### `JULIA_EXCLUSIVE`
 
 If set to anything besides `0`, then Julia's thread policy is consistent with

doc/src/manual/multi-threading.md

@@ -72,6 +72,15 @@ julia> Threads.threadid()
     three processes have 2 threads enabled. For more fine grained control over worker
     threads use [`addprocs`](@ref) and pass `-t`/`--threads` as `exeflags`.
 
+### Multiple GC Threads
+
+The Garbage Collector (GC) can use multiple threads. The amount used is either half the number
+of compute worker threads or configured by either the `--gcthreads` command line argument or by using the
+[`JULIA_NUM_GC_THREADS`](@ref env-gc-threads) environment variable.
+
+!!! compat "Julia 1.10"
+    The `--gcthreads` command line argument requires at least Julia 1.10.
+
 ## [Threadpools](@id man-threadpools)
 
 When a program's threads are busy with many tasks to run, tasks may experience

src/Makefile

@@ -99,7 +99,7 @@ ifeq ($(USE_SYSTEM_LIBUV),0)
 UV_HEADERS += uv.h
 UV_HEADERS += uv/*.h
 endif
-PUBLIC_HEADERS := $(BUILDDIR)/julia_version.h $(wildcard $(SRCDIR)/support/*.h) $(addprefix $(SRCDIR)/,julia.h julia_assert.h julia_threads.h julia_fasttls.h julia_locks.h julia_atomics.h jloptions.h)
+PUBLIC_HEADERS := $(BUILDDIR)/julia_version.h $(wildcard $(SRCDIR)/support/*.h) $(addprefix $(SRCDIR)/,work-stealing-queue.h julia.h julia_assert.h julia_threads.h julia_fasttls.h julia_locks.h julia_atomics.h jloptions.h)
 ifeq ($(OS),WINNT)
 PUBLIC_HEADERS += $(addprefix $(SRCDIR)/,win32_ucontext.h)
 endif