Re: [PATCH 6/6] mm/page_alloc: Introduce vm.percpu_pagelist_high_fraction

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, May 28, 2021 at 01:59:37PM +0200, Vlastimil Babka wrote:
> On 5/25/21 10:01 AM, Mel Gorman wrote:
> > This introduces a new sysctl vm.percpu_pagelist_high_fraction. It is
> > similar to the old vm.percpu_pagelist_fraction. The old sysctl increased
> > both pcp->batch and pcp->high with the higher pcp->high potentially
> > reducing zone->lock contention. However, the higher pcp->batch value also
> > potentially increased allocation latency while the PCP was refilled.
> > This sysctl only adjusts pcp->high so that zone->lock contention is
> > potentially reduced but allocation latency during a PCP refill remains
> > the same.
> > 
> >   # grep -E "high:|batch" /proc/zoneinfo | tail -2
> >               high:  649
> >               batch: 63
> > 
> >   # sysctl vm.percpu_pagelist_high_fraction=8
> >   # grep -E "high:|batch" /proc/zoneinfo | tail -2
> >               high:  35071
> >               batch: 63
> > 
> >   # sysctl vm.percpu_pagelist_high_fraction=64
> >               high:  4383
> >               batch: 63
> > 
> >   # sysctl vm.percpu_pagelist_high_fraction=0
> >               high:  649
> >               batch: 63
> > 
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> > Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 

Thanks.

> Documentation nit below:
> 
> > @@ -789,6 +790,25 @@ panic_on_oom=2+kdump gives you very strong tool to investigate
> >  why oom happens. You can get snapshot.
> >  
> >  
> > +percpu_pagelist_high_fraction
> > +=============================
> > +
> > +This is the fraction of pages in each zone that are allocated for each
> > +per cpu page list.  The min value for this is 8.  It means that we do
> > +not allow more than 1/8th of pages in each zone to be allocated in any
> > +single per_cpu_pagelist.
> 
> This, while technically correct (as an upper limit) is somewhat misleading as
> the limit for a single per_cpu_pagelist also considers the number of local cpus.
> 
> >  This entry only changes the value of hot per
> > +cpu pagelists. User can specify a number like 100 to allocate 1/100th
> > +of each zone to each per cpu page list.
> 
> This is worse. Anyone trying to reproduce this example on a system with multiple
> cpus per node and checking the result will be puzzled.
> So I think the part about number of local cpus should be mentioned to avoid
> confusion.
> 

Is this any better?

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index e85c2f21d209..2da25735a629 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -793,15 +793,16 @@ why oom happens. You can get snapshot.
 percpu_pagelist_high_fraction
 =============================
 
-This is the fraction of pages in each zone that are allocated for each
-per cpu page list.  The min value for this is 8.  It means that we do
-not allow more than 1/8th of pages in each zone to be allocated in any
-single per_cpu_pagelist.  This entry only changes the value of hot per
-cpu pagelists. User can specify a number like 100 to allocate 1/100th
-of each zone to each per cpu page list.
-
-The batch value of each per cpu pagelist remains the same regardless of the
-value of the high fraction so allocation latencies are unaffected.
+This is the fraction of pages in each zone that are can be stored to
+per-cpu page lists. It is an upper boundary that is divided depending
+on the number of online CPUs. The min value for this is 8 which means
+that we do not allow more than 1/8th of pages in each zone to be stored
+on per-cpu page lists. This entry only changes the value of hot per-cpu
+page lists. A user can specify a number like 100 to allocate 1/100th of
+each zone between per-cpu lists.
+
+The batch value of each per-cpu page list remains the same regardless of
+the value of the high fraction so allocation latencies are unaffected.
 
 The initial value is zero. Kernel uses this value to set the high pcp->high
 mark based on the low watermark for the zone and the number of local
-- 
Mel Gorman
SUSE Labs