shithub: riscv

Info  •  Files  •  Log  •  Branches

kernel: halt idle processors on mp system by default (from sources)

one can add:

int idle_spin = 1;

in the kernel configuration to enable the old behaviour. see the
comment at idlehands().

--- a/sys/src/9/pc/main.c

+++ b/sys/src/9/pc/main.c

@@ -31,6 +31,7 @@

 int nconf;

 uchar *sp;	/* user stack of init proc */

 int delaylink;

+int idle_spin;

 static void

 multibootargs(void)

@@ -1083,6 +1084,15 @@

 /*

  *  put the processor in the halt state if we've no processes to run.

  *  an interrupt will get us going again.

+ *

+ *  halting in an smp system can result in a startup latency for

+ *  processes that become ready.

+ *  if idle_spin is zero, we care more about saving energy

+ *  than reducing this latency.

+ *

+ *  the performance loss with idle_spin == 0 seems to be slight

+ *  and it reduces lock contention (thus system time and real time)

+ *  on many-core systems with large values of NPROC.

  */

 void

 idlehands(void)

@@ -1093,4 +1103,6 @@

 		halt();

 	else if(m->cpuidcx & Monitor)

 		mwait(&nrdy);

+	else if(idle_spin == 0)

+		halt();

 }

-- 

⑨