Add Mmap.madvise!

NEWS.md

@@ -147,6 +147,10 @@ Standard library changes
 * Change `uuid1` and `uuid4` to use `Random.RandomDevice()` as default random number generator ([#35872]).
 * Added `parse(::Type{UUID}, ::AbstractString)` method
 
+#### Mmap
+* On Unix systems, the `Mmap.madvise!` function (along with OS-specific `Mmap.MADV_*`
+  constants) has been added to give advice on handling of memory-mapped arrays. ([#37369])
+
 Deprecated or removed
 ---------------------
 

stdlib/Mmap/src/Mmap.jl

@@ -335,10 +335,10 @@ const MS_SYNC = 4
 Forces synchronization between the in-memory version of a memory-mapped `Array` or
 [`BitArray`](@ref) and the on-disk version.
 """
-function sync!(m::Array{T}, flags::Integer=MS_SYNC) where T
+function sync!(m::Array, flags::Integer=MS_SYNC)
     offset = rem(UInt(pointer(m)), PAGESIZE)
     ptr = pointer(m) - offset
-    mmaplen = length(m) * sizeof(T) + offset
+    mmaplen = sizeof(m) + offset
     GC.@preserve m @static if Sys.isunix()
         systemerror("msync",
                     ccall(:msync, Cint, (Ptr{Cvoid}, Csize_t, Cint), ptr, mmaplen, flags) != 0)
@@ -349,4 +349,64 @@ function sync!(m::Array{T}, flags::Integer=MS_SYNC) where T
 end
 sync!(B::BitArray, flags::Integer=MS_SYNC) = sync!(B.chunks, flags)
 
+@static if Sys.isunix()
+const MADV_NORMAL = 0
+const MADV_RANDOM = 1
+const MADV_SEQUENTIAL = 2
+const MADV_WILLNEED = 3
+const MADV_DONTNEED = 4
+if Sys.islinux()
+    const MADV_FREE = 8
+    const MADV_REMOVE = 9
+    const MADV_DONTFORK = 10
+    const MADV_DOFORK = 11
+    const MADV_MERGEABLE = 12
+    const MADV_UNMERGEABLE = 13
+    const MADV_HUGEPAGE = 14
+    const MADV_NOHUGEPAGE = 15
+    const MADV_DONTDUMP = 16
+    const MADV_DODUMP = 17
+    const MADV_WIPEONFORK = 18
+    const MADV_KEEPONFORK = 19
+    const MADV_COLD = 20
+    const MADV_PAGEOUT = 21
+    const MADV_HWPOISON = 100
+    const MADV_SOFT_OFFLINE = 101
+elseif Sys.isapple()
+    const MADV_FREE = 5
+elseif Sys.isfreebsd() || Sys.isdragonfly()
+    const MADV_FREE = 5
+    const MADV_NOSYNC = 6
+    const MADV_AUTOSYNC = 7
+    const MADV_NOCORE = 8
+    const MADV_CORE = 9
+    if Sys.isfreebsd()
+        const MADV_PROTECT = 10
+    else
+        const MADV_INVAL = 10
+        const MADV_SETMAP = 11
+    end
+elseif Sys.isopenbsd() || Sys.isnetbsd()
+    const MADV_SPACEAVAIL = 5
+    const MADV_FREE = 6
+end
+
+"""
+    Mmap.madvise!(array, flag::Integer = Mmap.MADV_NORMAL)
+
+Advises the kernel on the intended usage of the memory-mapped `array`, with the intent
+`flag` being one of the available `MADV_*` constants.
+"""
+function madvise!(m::Array, flag::Integer=MADV_NORMAL)
+    offset = rem(UInt(pointer(m)), PAGESIZE)
+    ptr = pointer(m) - offset
+    mmaplen = sizeof(m) + offset
+    GC.@preserve m begin
+        systemerror("madvise",
+                    ccall(:madvise, Cint, (Ptr{Cvoid}, Csize_t, Cint), ptr, mmaplen, flag) != 0)
+    end
+end
+madvise!(B::BitArray, flag::Integer=MADV_NORMAL) = madvise!(B.chunks, flag)
+end # Sys.isunix()
+
 end # module

stdlib/Mmap/test/runtests.jl

@@ -307,6 +307,20 @@ n = similar(m, 12)
 @test size(n) == (12,)
 finalize(m); m = nothing; GC.gc()
 
+if Sys.isunix()
+    file = tempname()
+    write(file, rand(Float64, 20))
+    A = Mmap.mmap(file, Vector{Float64}, 20)
+    @test Mmap.madvise!(A, Mmap.MADV_WILLNEED) === nothing # checking for no error
+    finalize(A); A = nothing; GC.gc()
+
+    write(file, BitArray(rand(Bool, 20)))
+    b = Mmap.mmap(file, BitArray, 20)
+    @test Mmap.madvise!(b, Mmap.MADV_WILLNEED) === nothing
+    finalize(b); b = nothing; GC.gc()
+    rm(file)
+end
+
 # test #14885
 file = tempname()
 touch(file)