From c1635ad8f0bb9fbe5bfbf0a633c78a03930758c4 Mon Sep 17 00:00:00 2001 From: Shenghou Ma Date: Mon, 30 Sep 2019 09:44:37 -0400 Subject: [PATCH] runtime: fix darwin syscall performance regression While understanding why syscall.Read is 2x slower on darwin/amd64, I found out that, contrary to popular belief, the slowdown is not due to the migration to use libSystem.dylib instead of direct SYSCALLs, i.e., CL 141639 (and #17490), but due to a subtle change introduced in CL 141639. Previously, syscall.Read used syscall.Syscall(SYS_READ), whose preamble called runtime.entersyscall, but after CL 141639, syscall.Read changes to call runtime.syscall_syscall instead, which in turn calls runtime.entersyscallblock instead of runtime.entersyscall. And the entire 2x slow down can be attributed to this change. I think this is unnecessary as even though syscalls like Read might block, it does not always block, so there is no need to handoff P proactively for each Read. Additionally, we have been fine with not handing off P for each Read prior to Go 1.12, so we probably don't need to change it. This changes restores the pre-Go 1.12 behavior, where syscall preamble uses runtime.entersyscall, and we rely on sysmon to take P back from g blocked in syscalls. Change-Id: If76e97b5a7040cf1c10380a567c4f5baec3121ba Reviewed-on: https://go-review.googlesource.com/c/go/+/197938 Run-TryBot: Minux Ma TryBot-Result: Gobot Gobot Reviewed-by: Keith Randall Reviewed-by: Ian Lance Taylor --- src/runtime/sys_darwin.go | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/src/runtime/sys_darwin.go b/src/runtime/sys_darwin.go index 932a2a7757..46825d5937 100644 --- a/src/runtime/sys_darwin.go +++ b/src/runtime/sys_darwin.go @@ -60,7 +60,7 @@ func libcCall(fn, arg unsafe.Pointer) int32 { //go:nosplit //go:cgo_unsafe_args func syscall_syscall(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) { - entersyscallblock() + entersyscall() libcCall(unsafe.Pointer(funcPC(syscall)), unsafe.Pointer(&fn)) exitsyscall() return @@ -71,7 +71,7 @@ func syscall() //go:nosplit //go:cgo_unsafe_args func syscall_syscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr) { - entersyscallblock() + entersyscall() libcCall(unsafe.Pointer(funcPC(syscall6)), unsafe.Pointer(&fn)) exitsyscall() return @@ -82,7 +82,7 @@ func syscall6() //go:nosplit //go:cgo_unsafe_args func syscall_syscall6X(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr) { - entersyscallblock() + entersyscall() libcCall(unsafe.Pointer(funcPC(syscall6X)), unsafe.Pointer(&fn)) exitsyscall() return @@ -93,7 +93,7 @@ func syscall6X() //go:nosplit //go:cgo_unsafe_args func syscall_syscallPtr(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) { - entersyscallblock() + entersyscall() libcCall(unsafe.Pointer(funcPC(syscallPtr)), unsafe.Pointer(&fn)) exitsyscall() return -- 2.50.0