win: `uv_available_parallelism` doesn't respect process affinity

[giordano]

Sorry, this is a full Julia report, but it involves only libuv functions. The problem is that on Windows, but not on Linux, uv_available_parallelism doesn't respect process affinity, contrary to uv_thread_getaffinity, returning total number of logical threads available in the system:

julia> run(setcpuaffinity(`$(Base.julia_cmd()) -L $(joinpath(Sys.BINDIR, Base.DATAROOTDIR, "julia", "test", "print_process_affinity.jl")) -e '@show(length(uv_thread_getaffinity())); @show(@ccall uv_available_parallelism()::Cint); @show(Sys.CPU_THREADS)'`, 1:2));
length(uv_thread_getaffinity()) = 2
#= none:1 =# @ccall(uv_available_parallelism()::Cint) = 4
Sys.CPU_THREADS = 4

I'm using setcpuaffinity to set the affinity of the process and limit it two cores, I'm loading the test/print_process_affinity.jl script which defines a small wrapper around uv_thread_getaffinity, and I'm also printing Julia's Sys.CPU_THREAD to show the total number of logical threads on the system.

On Linux I get

julia> run(setcpuaffinity(`$(Base.julia_cmd()) -L $(joinpath(Sys.BINDIR, Base.DATAROOTDIR, "julia", "test", "print_process_affinity.jl")) -e '@show(length(uv_thread_getaffinity())); @show(@ccall uv_available_parallelism()::Cint); @show(Sys.CPU_THREADS)'`, 1:2));
length(uv_thread_getaffinity()) = 2
#= none:1 =# @ccall(uv_available_parallelism()::Cint) = 2
Sys.CPU_THREADS = 32

as expected.

Current build of libuv used in Julia is based on libuv v1.48.0. CC: @vtjnash.

[bnoordhuis]

The Windows implementation of uv_available_parallelism() is pretty simplistic at the moment; it calls GetSystemInfo() and returns the dwNumberOfProcessors field.

Are you saying it should use GetProcessAffinityMask() instead?

[giordano]

Are you saying it should use GetProcessAffinityMask() instead?

I guess so, since the Linux implementation seems to do the equivalent of that, and I believe it'd more closely match the "available" parallelism.

[bnoordhuis]

Does this patch work like you expect it to?

diff --git a/src/win/util.c b/src/win/util.c
index deed2e35..c667eea8 100644
--- a/src/win/util.c
+++ b/src/win/util.c
@@ -512,19 +512,22 @@ int uv_uptime(double* uptime) {
 
 
 unsigned int uv_available_parallelism(void) {
-  SYSTEM_INFO info;
-  unsigned rc;
+  DWORD_PTR procmask;
+  int count;
+  int i;
 
   /* TODO(bnoordhuis) Use GetLogicalProcessorInformationEx() to support systems
    * with > 64 CPUs? See https://github.com/libuv/libuv/pull/3458
    */
-  GetSystemInfo(&info);
+  count = 0;
+  if (GetProcessAffinityMask(GetCurrentProcess(), &procmask, NULL))
+    for (i = 0; i < 64; i++)  /* a.k.a. count = popcount(procmask); */
+      count += 1 & (procmask >> i);
 
-  rc = info.dwNumberOfProcessors;
-  if (rc < 1)
-    rc = 1;
+  if (count > 0)
+    return count;
 
-  return rc;
+  return 1;
 }
 
 

[giordano]

I can't test that exactly because I don't have a Windows environment set up with a compiler to build libuv and Julia, but I can test the syscalls from julia:

julia> procmask = Ref(UInt64(0))
Base.RefValue{UInt64}(0x0000000000000000)

julia> sysmask = Ref(UInt64(0))
Base.RefValue{UInt64}(0x0000000000000000)

julia> @ccall GetProcessAffinityMask(@ccall(GetCurrentProcess()::Ptr{Cvoid})::Ptr{Cvoid}, procmask::Ptr{UInt64}, sysmask::Ptr{UInt64})::Cint
1

julia> count_ones(procmask[])
4

julia> run(setcpuaffinity(`julia -q`, 1:2));
julia> procmask = Ref(UInt64(0))
Base.RefValue{UInt64}(0x0000000000000000)

julia> sysmask = Ref(UInt64(0))
Base.RefValue{UInt64}(0x0000000000000000)

julia> @ccall GetProcessAffinityMask(@ccall(GetCurrentProcess()::Ptr{Cvoid})::Ptr{Cvoid}, procmask::Ptr{UInt64}, sysmask::Ptr{UInt64})::Cint
1

julia> count_ones(procmask[])
2

In the outer process (without restrictions), procmask has 4 non-zero values, while in the process with the restricted affinity there are only 2 non-zero values, so I'd say that'd work for me, yes, thanks!