add -maxparallelism flag (default 1/2 NumCPU)

This change adds a `-maxparallelism` flag which defaults to 1/2 the NumCPU
on the machine. It is used to control the language server's use of
parallelism in editor environments where using all available resources to
answer a query is not most ideal.

Author: Stephen Gutekanst

langserver/symbol.go

@@ -340,6 +340,12 @@ func (h *LangHandler) handleWorkspaceSymbol(ctx context.Context, conn jsonrpc2.J
 	return h.handleSymbol(ctx, conn, req, q, params.Limit)
 }
 
+// MaxParallelism controls the maximum number of goroutines that should be used
+// to fulfill requests. This is useful in editor environments where users do
+// not want results ASAP, but rather just semi quickly without eating all of
+// their CPU.
+var MaxParallelism = 8
+
 func (h *LangHandler) handleSymbol(ctx context.Context, conn jsonrpc2.JSONRPC2, req *jsonrpc2.Request, query Query, limit int) ([]lsp.SymbolInformation, error) {
 	results := resultSorter{Query: query, results: make([]scoredSymbol, 0)}
 	{

main.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"net/http"
 	"os"
+	"runtime"
 	"runtime/debug"
 	"time"
 
@@ -27,6 +28,7 @@ var (
 	pprof             = flag.String("pprof", ":6060", "start a pprof http server (https://golang.org/pkg/net/http/pprof/)")
 	freeosmemory      = flag.Bool("freeosmemory", true, "aggressively free memory back to the OS")
 	usebinarypkgcache = flag.Bool("usebinarypkgcache", true, "use $GOPATH/pkg binary .a files (improves performance)")
+	maxparallelism    = flag.Int("maxparallelism", -1, "use at max N parallel goroutines to fulfill requests")
 )
 
 // version is the version field we report back. If you are releasing a new version:
@@ -52,6 +54,15 @@ func main() {
 	}
 	langserver.UseBinaryPkgCache = *usebinarypkgcache
 
+	// Default max parallelism to half the CPU cores, but at least always one.
+	if *maxparallelism <= 0 {
+		*maxparallelism = runtime.NumCPU() / 2
+		if *maxparallelism <= 0 {
+			*maxparallelism = 1
+		}
+	}
+	langserver.MaxParallelism = *maxparallelism
+
 	if err := run(); err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)