Messages in this thread | | | Subject | Re: [PATCH 03/10] lib: dampen the percpu_counter FBC_BATCH | From | Peter Zijlstra <> | Date | Sat, 21 Apr 2007 12:58:57 +0200 |
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On Sat, 2007-04-21 at 02:55 -0700, Andrew Morton wrote: > On Fri, 20 Apr 2007 17:51:57 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > With the current logic the percpu_counter's accuracy delta is quadric > > wrt the number of cpus in the system, reduce this to O(n ln n). > > > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> > > --- > > include/linux/percpu_counter.h | 7 ++----- > > 1 file changed, 2 insertions(+), 5 deletions(-) > > > > Index: linux-2.6-mm/include/linux/percpu_counter.h > > =================================================================== > > --- linux-2.6-mm.orig/include/linux/percpu_counter.h > > +++ linux-2.6-mm/include/linux/percpu_counter.h > > @@ -11,6 +11,7 @@ > > #include <linux/threads.h> > > #include <linux/percpu.h> > > #include <linux/types.h> > > +#include <linux/log2.h> > > > > #ifdef CONFIG_SMP > > > > @@ -20,11 +21,7 @@ struct percpu_counter { > > s32 *counters; > > }; > > > > -#if NR_CPUS >= 16 > > -#define FBC_BATCH (NR_CPUS*2) > > -#else > > -#define FBC_BATCH (NR_CPUS*4) > > -#endif > > +#define FBC_BATCH (8*ilog2(NR_CPUS)) > > > > static inline void percpu_counter_init(struct percpu_counter *fbc, s64 amount) > > { > > I worry that this might be too small when there are hundreds of CPUs online. > > With 1024 CPUs we go for the lock once per 80 counts. That's not much. > > If we have 1024 CPUs, each one of which is incrementing this counter at N > Hz, we have 1024/80=12 CPUs all going for the same lock at N Hz. It could > get bad. > > But I don't know what the gain is for this loss. Your changelog should > have told us. > > What problem is this patch solving?
In 10/10 I introduce bdi_stat_delta() which gives the maximum error of a single counter. That is used to switch between precise (percpu_counter_sum) and imprecise (percpu_counter_read) accesses of the stats.
I worried that the current quadric error would be too large; and as the ZVC counters also use a logarithmic error bound I thought it would be good to have here as well.
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