cvs commit: src/sys/kern kern_mutex.c

[John Baldwin]

On Thursday 07 June 2007 04:15:13 am Bruce Evans wrote:
> On Thu, 7 Jun 2007, Bruce Evans wrote:
> 
> > (Cold client):
> >      834.39 real      1300.21 user       192.19 sys
> >   1323006  voluntary context switches
> >   1526348  involuntary context switches
> > ...
> > This is with 4BSD, no PREEMPTION, and pagezero disabled.  With the
> > ...
> > The next run will have pagezero resetting its priority when this priority
> > gets clobbered.
> 
> That gave only mainly more voluntary context switches (13.5+ million instead
> of the best observed value of 1.3+ million or the value of 2.9+ million
> without priority resetting.  It reduced the pagezero time from 30 seconds to
> 24.  It didn't change the real time significantly.

Hmm, one problem with the non-preemption page zero is that it doesn't yield 
the lock when it voluntarily yields.  I wonder if something like this patch 
would help things for the non-preemption case:

Index: vm_zeroidle.c
===================================================================
RCS file: /usr/cvs/src/sys/vm/vm_zeroidle.c,v
retrieving revision 1.45
diff -u -r1.45 vm_zeroidle.c
--- vm_zeroidle.c       18 May 2007 07:10:50 -0000      1.45
+++ vm_zeroidle.c       7 Jun 2007 14:56:02 -0000
@@ -147,8 +147,10 @@
 #ifndef PREEMPTION
                        if (sched_runnable()) {
                                mtx_lock_spin(&sched_lock);
+                               mtx_unlock(&vm_page_queue_free_mtx);
                                mi_switch(SW_VOL, NULL);
                                mtx_unlock_spin(&sched_lock);
+                               mtx_lock(&vm_page_queue_free_mtx);
                        }
 #endif
                } else {


We could simulate this behavior some by using a critical section to control 
when preemptions happen so that we wait until we drop the lock perhaps to 
allow preemptions.  Something like this:

> Index: vm_zeroidle.c
===================================================================
RCS file: /usr/cvs/src/sys/vm/vm_zeroidle.c,v
retrieving revision 1.45
diff -u -r1.45 vm_zeroidle.c
--- vm_zeroidle.c       18 May 2007 07:10:50 -0000      1.45
+++ vm_zeroidle.c       7 Jun 2007 14:58:39 -0000
@@ -110,8 +110,10 @@
        if (m != NULL && (m->flags & PG_ZERO) == 0) {
                vm_pageq_remove_nowakeup(m);
                mtx_unlock(&vm_page_queue_free_mtx);
+               critical_exit();
                pmap_zero_page_idle(m);
                mtx_lock(&vm_page_queue_free_mtx);
+               critical_enter();
                m->flags |= PG_ZERO;
                vm_pageq_enqueue(PQ_FREE + m->pc, m);
                ++vm_page_zero_count;
@@ -141,20 +143,25 @@
        idlezero_enable = idlezero_enable_default;

        mtx_lock(&vm_page_queue_free_mtx);
+       critical_enter();
        for (;;) {
                if (vm_page_zero_check()) {
                        vm_page_zero_idle();
 #ifndef PREEMPTION
                        if (sched_runnable()) {
                                mtx_lock_spin(&sched_lock);
+                               mtx_unlock(&vm_page_queue_free_mtx);
                                mi_switch(SW_VOL, NULL);
                                mtx_unlock_spin(&sched_lock);
+                               mtx_lock(&vm_page_queue_free_mtx);
                        }
 #endif
                } else {
+                       critical_exit();
                        wakeup_needed = TRUE;
                        msleep(&zero_state, &vm_page_queue_free_mtx, 0,
                            "pgzero", hz * 300);
+                       critical_enter();
                }
        }
 }


-- 
John Baldwin