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SubjectRe: 2.4.4 sluggish under fork load
On Mon, Apr 30, 2001 at 11:38:23PM -0300, Rik van Riel wrote:
> On Mon, 30 Apr 2001, Andrea Arcangeli wrote:
> > On Sun, Apr 29, 2001 at 10:26:57AM +0200, Peter Osterlund wrote:
>
> > > - p->counter = current->counter;
> > > - current->counter = 0;
> > > + p->counter = (current->counter + 1) >> 1;
> > > + current->counter >>= 1;
> > > + current->policy |= SCHED_YIELD;
> > > current->need_resched = 1;
> >
> > please try to reproduce the bad behaviour with 2.4.4aa2. There's a bug
> > in the parent-timeslice patch in 2.4 that I fixed while backporting it
> > to 2.2aa and that I now forward ported the fix to 2.4aa. The fact
> > 2.4.4 gives the whole timeslice to the child just gives more light to
> > such bug.
>
> The fact that 2.4.4 gives the whole timeslice to the child
> is just bogus to begin with.
>
> The problem people tried to solve was "make sure the kernel
> runs the child first after a fork", this has just about
> NOTHING to do with how the timeslice is distributed.
>
> Now, since we are in a supposedly stable branch of the kernel,
> why mess with the timeslice distribution between parent and
> child? The timeslice distribution that has worked very well
> for the last YEARS...

I'm running with this below patch applied since a some time (I didn't
submitted it because for some reason unless I do p->policy &=
~SCHED_YIELD ksoftirqd deadlocks at boot and I didn't yet investigated
why, and I'd like to have the whole picture on it first):

diff -urN z/include/linux/sched.h z1/include/linux/sched.h
--- z/include/linux/sched.h Mon Apr 30 04:22:25 2001
+++ z1/include/linux/sched.h Mon Apr 30 02:45:07 2001
@@ -301,7 +301,7 @@
* all fields in a single cacheline that are needed for
* the goodness() loop in schedule().
*/
- int counter;
+ volatile int counter;
int nice;
unsigned int policy;
struct mm_struct *mm;
diff -urN z/kernel/fork.c z1/kernel/fork.c
--- z/kernel/fork.c Mon Apr 30 04:22:25 2001
+++ z1/kernel/fork.c Mon Apr 30 03:49:26 2001
@@ -666,17 +666,17 @@
p->pdeath_signal = 0;

/*
- * Give the parent's dynamic priority entirely to the child. The
- * total amount of dynamic priorities in the system doesn't change
- * (more scheduling fairness), but the child will run first, which
- * is especially useful in avoiding a lot of copy-on-write faults
- * if the child for a fork() just wants to do a few simple things
- * and then exec(). This is only important in the first timeslice.
- * In the long run, the scheduling behavior is unchanged.
+ * Scheduling the child first is especially useful in avoiding a
+ * lot of copy-on-write faults if the child for a fork() just wants
+ * to do a few simple things and then exec().
*/
- p->counter = current->counter;
- current->counter = 0;
- current->need_resched = 1;
+ {
+ int counter = current->counter >> 1;
+ current->counter = p->counter = counter;
+ p->policy &= ~SCHED_YIELD;
+ current->policy |= SCHED_YIELD;
+ current->need_resched = 1;
+ }
/* Tell the parent if it can get back its timeslice when child exits */
p->get_child_timeslice = 1;

The only point of my previous email is that if a fork loop has very
invasive effect on the rest of the system that more probably indicates
people got bitten by the bug in the parent-timeslice logic, furthmore I
never noticed any sluggish behaviour on my systems and before posting my
previous email I had 1 definitive feedback that the bad beahviour
observed on vanilla 2.4.4 with parallel compiles in the background got
cured *completly* by my tree (that in the tested revision didn't
included the above inlined change yet). So I thought it was worth
mentioning about the effect of the parent-timeslice bugfix here too.
This doesn't mean I don't want something like the above inlined patch
integrated.

Andrea
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