runtime, cmd/link: optimize memory allocation on wasm

WebAssembly's memory is contiguous. Allocating memory at a high address
also allocates all memory up to that address. This change reduces
the initial memory allocated on wasm from 1GB to 16MB by using multiple
heap arenas and reducing the size of a heap arena.

Fixes #27462.

Change-Id: Ic941e6edcadd411e65a14cb2f9fd6c8eae02fc7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/170950
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/cmd/link/internal/wasm/asm.go b/src/cmd/link/internal/wasm/asm.go
index abb4409188f29004468aa1849ee54b7d3a68661f..ea6b406c7e217dcd064ca5878468c5bd5504c22b 100644 (file)
--- a/src/cmd/link/internal/wasm/asm.go
+++ b/src/cmd/link/internal/wasm/asm.go
@@ -297,18 +297,18 @@ func writeTableSec(ctxt *ld.Link, fns []*wasmFunc) {
 }
 
 // writeMemorySec writes the section that declares linear memories. Currently one linear memory is being used.
+// Linear memory always starts at address zero. More memory can be requested with the GrowMemory instruction.
 func writeMemorySec(ctxt *ld.Link) {
        sizeOffset := writeSecHeader(ctxt, sectionMemory)
 
-       // Linear memory always starts at address zero.
-       // The unit of the sizes is "WebAssembly page size", which is 64Ki.
-       // The minimum is currently set to 1GB, which is a lot.
-       // More memory can be requested with the grow_memory instruction,
-       // but this operation currently is rather slow, so we avoid it for now.
-       // TODO(neelance): Use lower initial memory size.
-       writeUleb128(ctxt.Out, 1)       // number of memories
-       ctxt.Out.WriteByte(0x00)        // no maximum memory size
-       writeUleb128(ctxt.Out, 1024*16) // minimum (initial) memory size
+       const (
+               initialSize  = 16 << 20 // 16MB, enough for runtime init without growing
+               wasmPageSize = 64 << 10 // 64KB
+       )
+
+       writeUleb128(ctxt.Out, 1)                        // number of memories
+       ctxt.Out.WriteByte(0x00)                         // no maximum memory size
+       writeUleb128(ctxt.Out, initialSize/wasmPageSize) // minimum (initial) memory size
 
        writeSecSize(ctxt, sizeOffset)
 }

diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index 9feec1b007ef1630639e8b1a1b925956233478d8..c22c7aa7dc1f6675c74acfd20b918b5bc728f4b4 100644 (file)
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -248,7 +248,7 @@ const (
        // logHeapArenaBytes is log_2 of heapArenaBytes. For clarity,
        // prefer using heapArenaBytes where possible (we need the
        // constant to compute some other constants).
-       logHeapArenaBytes = (6+20)*(_64bit*(1-sys.GoosWindows)*(1-sys.GoosAix)) + (2+20)*(_64bit*sys.GoosWindows) + (2+20)*(1-_64bit) + (8+20)*sys.GoosAix
+       logHeapArenaBytes = (6+20)*(_64bit*(1-sys.GoosWindows)*(1-sys.GoosAix)*(1-sys.GoarchWasm)) + (2+20)*(_64bit*sys.GoosWindows) + (2+20)*(1-_64bit) + (8+20)*sys.GoosAix + (2+20)*sys.GoarchWasm
 
        // heapArenaBitmapBytes is the size of each heap arena's bitmap.
        heapArenaBitmapBytes = heapArenaBytes / (sys.PtrSize * 8 / 2)
@@ -394,7 +394,7 @@ func mallocinit() {
        _g_.m.mcache = allocmcache()
 
        // Create initial arena growth hints.
-       if sys.PtrSize == 8 && GOARCH != "wasm" {
+       if sys.PtrSize == 8 {
                // On a 64-bit machine, we pick the following hints
                // because:
                //