add ability to specify heap size hint as a percent

doc/man/julia.1

@@ -227,8 +227,8 @@ fallbacks to the latest compatible BugReporting.jl if not. For more information,
 
 .TP
 --heap-size-hint=<size>
-Forces garbage collection if memory usage is higher than that value. The memory hint might be
-specified in megabytes (500M) or gigabytes (1.5G)
+Forces garbage collection if memory usage is higher than that value. The value can be
+specified in units of K, M, G, T, or % of physical memory.
 
 .TP
 --compile={yes*|no|all|min}

src/gc.c

@@ -4037,14 +4037,22 @@ void jl_gc_init(void)
     gc_num.max_pause = 0;
     gc_num.max_memory = 0;
 
+    uint64_t mem_reserve = 250*1024*1024; // LLVM + other libraries need some amount of memory
+    uint64_t min_heap_size_hint = mem_reserve + 1*1024*1024;
+    uint64_t hint = jl_options.heap_size_hint;
 #ifdef _P64
     total_mem = uv_get_total_memory();
-    uint64_t constrained_mem = uv_get_constrained_memory();
-    if (constrained_mem > 0 && constrained_mem < total_mem)
-        jl_gc_set_max_memory(constrained_mem - 250*1024*1024); // LLVM + other libraries need some amount of memory
+    if (hint == 0) {
+        uint64_t constrained_mem = uv_get_constrained_memory();
+        if (constrained_mem > 0 && constrained_mem < total_mem)
+            hint = constrained_mem;
+    }
 #endif
-    if (jl_options.heap_size_hint)
-        jl_gc_set_max_memory(jl_options.heap_size_hint - 250*1024*1024);
+    if (hint) {
+        if (hint < min_heap_size_hint)
+            hint = min_heap_size_hint;
+        jl_gc_set_max_memory(hint - mem_reserve);
+    }
 
     t_start = jl_hrtime();
 }

src/jloptions.c

@@ -189,7 +189,7 @@ static const char opts[]  =
     "                            --bug-report=help.\n\n"
 
     " --heap-size-hint=<size>    Forces garbage collection if memory usage is higher than that value.\n"
-    "                            The memory hint might be specified in megabytes(500M) or gigabytes(1G)\n\n"
+    "                            The value can be specified in units of K, M, G, T, or % of physical memory.\n\n"
 ;
 
 static const char opts_hidden[]  =
@@ -821,14 +821,24 @@ JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)
                         case 'T':
                             multiplier <<= 40;
                             break;
+                        case '%':
+                            if (value > 100)
+                                jl_errorf("julia: invalid percentage specified in --heap-size-hint");
+                            uint64_t mem = uv_get_total_memory();
+                            uint64_t cmem = uv_get_constrained_memory();
+                            if (cmem > 0 && cmem < mem)
+                                mem = cmem;
+                            multiplier = mem/100;
+                            break;
                         default:
+                            jl_errorf("julia: invalid unit specified in --heap-size-hint");
                             break;
                     }
                     jl_options.heap_size_hint = (uint64_t)(value * multiplier);
                 }
             }
             if (jl_options.heap_size_hint == 0)
-                jl_errorf("julia: invalid argument to --heap-size-hint without memory size specified");
+                jl_errorf("julia: invalid memory size specified in --heap-size-hint");
 
             break;
         case opt_gc_threads:

test/cmdlineargs.jl

@@ -1043,8 +1043,26 @@ end
         @test lines[3] == "foo"
         @test lines[4] == "bar"
     end
-#heap-size-hint, we reserve 250 MB for non GC memory (llvm, etc.)
-@test readchomp(`$(Base.julia_cmd()) --startup-file=no --heap-size-hint=500M -e "println(@ccall jl_gc_get_max_memory()::UInt64)"`) == "$((500-250)*1024*1024)"
+end
+
+@testset "heap size hint" begin
+    #heap-size-hint, we reserve 250 MB for non GC memory (llvm, etc.)
+    @test readchomp(`$(Base.julia_cmd()) --startup-file=no --heap-size-hint=500M -e "println(@ccall jl_gc_get_max_memory()::UInt64)"`) == "$((500-250)*1024*1024)"
+
+    mem = ccall(:uv_get_total_memory, UInt64, ())
+    cmem = ccall(:uv_get_constrained_memory, UInt64, ())
+    if cmem > 0 && cmem < mem
+        mem = cmem
+    end
+    maxmem = parse(UInt64, readchomp(`$(Base.julia_cmd()) --startup-file=no --heap-size-hint=25% -e "println(@ccall jl_gc_get_max_memory()::UInt64)"`))
+    hint = max(mem÷4, 251*1024*1024) - 250*1024*1024
+    MAX32HEAP = 1536 * 1024 * 1024
+    if Int === Int32 && hint > MAX32HEAP
+        hint = MAX32HEAP
+    end
+    @test abs(Float64(maxmem) - hint)/maxmem < 0.05
+
+    @test readchomp(`$(Base.julia_cmd()) --startup-file=no --heap-size-hint=10M -e "println(@ccall jl_gc_get_max_memory()::UInt64)"`) == "$(1*1024*1024)"
 end
 
 ## `Main.main` entrypoint