Add -Nmax<n> RTS feature (#10728)

Added maximum core use based on processors
-Nmax<x> chooses the minimum of processor count or x
Added documentation.

Reviewed By: simonmar, thomie

Differential Revision: https://phabricator.haskell.org/D1650

1 files changed, 13 insertions, 4 deletions

diff --git a/rts/RtsFlags.c b/rts/RtsFlags.c
index 94572792de..bd60591f33 100644
--- a/rts/RtsFlags.c
+++ b/rts/RtsFlags.c
@@ -391,8 +391,9 @@ usage_text[] = {
 "",
 #endif /* DEBUG */
 #if defined(THREADED_RTS) && !defined(NOSMP)
-"  -N[<n>]   Use <n> processors (default: 1, -N alone determines",
-"            the number of processors to use automatically)",
+"  -N[<n>]    Use <n> processors (default: 1, -N alone determines",
+"             the number of processors to use automatically)",
+"  -Nmax[<n>] Use up to n processors automatically",
 "  -qg[<n>]  Use parallel GC only for generations >= <n>",
 "            (default: 0, -qg alone turns off parallel GC)",
 "  -qb[<n>]  Use load-balancing in the parallel GC only for generations >= <n>",
@@ -1041,13 +1042,21 @@ error = rtsTrue;
                 } else {
                     int nNodes;
                     OPTION_SAFE; /* but see extra checks below... */
-                    nNodes = strtol(rts_argv[arg]+2, (char **) NULL, 10);
+
+                    // <=n feature request ticket #10728
+                    if (strncmp("max", &rts_argv[arg][2], 3) == 0) {
+                      int proc = (int)getNumberOfProcessors();
+                      nNodes = strtol(rts_argv[arg]+5, (char **) NULL, 10);
+                      if (nNodes > proc) { nNodes = proc; }
+                    } else {
+                      nNodes = strtol(rts_argv[arg]+2, (char **) NULL, 10);
+                    }
                     if (nNodes <= 0) {
                       errorBelch("bad value for -N");
                       error = rtsTrue;
                     }
                     if (rtsOptsEnabled == RtsOptsSafeOnly &&
-                        nNodes > (int)getNumberOfProcessors()) {
+                      nNodes > (int)getNumberOfProcessors()) {
                       errorRtsOptsDisabled("Using large values for -N is not allowed by default. %s");
                       stg_exit(EXIT_FAILURE);
                     }