If the allocation order is not high, direct compaction does nothing.
Can we skip compaction here if order drops to zero?
--- a/mm/vmscan.c Thu Aug 15 17:47:26 2013
+++ b/mm/vmscan.c Thu Aug 15 17:48:58 2013
@@ -3034,7 +3034,7 @@ static unsigned long balance_pgdat(pg_da
* Compact if necessary and kswapd is reclaiming at least the
* high watermark number of pages as requsted
*/
- if (pgdat_needs_compaction && sc.nr_reclaimed > nr_attempted)
+ if (pgdat_needs_compaction && sc.nr_reclaimed > nr_attempted && order)
compact_pgdat(pgdat, order);
/*
--
On Thu, Aug 15, 2013 at 06:02:53PM +0800, Hillf Danton wrote:
> If the allocation order is not high, direct compaction does nothing.
> Can we skip compaction here if order drops to zero?
>
If the allocation order is not high then
pgdat_needs_compaction == (order > 0) == false == no calling compact_pdatt
In the case where order is reset to 0 due to fragmentation then it does
call compact_pgdat but it does no work due to the cc->order check in
__compact_pgdat.
--
Mel Gorman
SUSE Labs
Hey Mel,
On Thu, Aug 15, 2013 at 11:47:27AM +0100, Mel Gorman wrote:
> On Thu, Aug 15, 2013 at 06:02:53PM +0800, Hillf Danton wrote:
> > If the allocation order is not high, direct compaction does nothing.
> > Can we skip compaction here if order drops to zero?
> >
>
> If the allocation order is not high then
>
> pgdat_needs_compaction == (order > 0) == false == no calling compact_pdatt
>
> In the case where order is reset to 0 due to fragmentation then it does
> call compact_pgdat but it does no work due to the cc->order check in
> __compact_pgdat.
>
I am looking at mmotm-2013-08-07-16-55 but couldn't find cc->order
check right before compact_zone in __comact_pgdat.
Could you pinpoint code piece?
> --
> Mel Gorman
> SUSE Labs
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to [email protected]. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
--
Kind regards,
Minchan Kim
On Thu, Aug 15, 2013 at 10:41:39PM +0900, Minchan Kim wrote:
> Hey Mel,
>
> On Thu, Aug 15, 2013 at 11:47:27AM +0100, Mel Gorman wrote:
> > On Thu, Aug 15, 2013 at 06:02:53PM +0800, Hillf Danton wrote:
> > > If the allocation order is not high, direct compaction does nothing.
> > > Can we skip compaction here if order drops to zero?
> > >
> >
> > If the allocation order is not high then
> >
> > pgdat_needs_compaction == (order > 0) == false == no calling compact_pdatt
> >
> > In the case where order is reset to 0 due to fragmentation then it does
> > call compact_pgdat but it does no work due to the cc->order check in
> > __compact_pgdat.
> >
>
> I am looking at mmotm-2013-08-07-16-55 but couldn't find cc->order
> check right before compact_zone in __comact_pgdat.
> Could you pinpoint code piece?
>
Thanks, I screwed up as that check happens too late. However, it still
ends up not mattering because it does this
compact_pgdat
-> __compact_pgdat
-> compact_zone
-> compaction_suitable
For order == 0, compaction_suitable will return either COMPACT_SKIPPED
(if the watermarks are not met) and COMPACT_PARTIAL otherwise. Either
way, compaction doesn't run.
--
Mel Gorman
SUSE Labs
On Thu, Aug 15, 2013 at 02:56:27PM +0100, Mel Gorman wrote:
> On Thu, Aug 15, 2013 at 10:41:39PM +0900, Minchan Kim wrote:
> > Hey Mel,
> >
> > On Thu, Aug 15, 2013 at 11:47:27AM +0100, Mel Gorman wrote:
> > > On Thu, Aug 15, 2013 at 06:02:53PM +0800, Hillf Danton wrote:
> > > > If the allocation order is not high, direct compaction does nothing.
> > > > Can we skip compaction here if order drops to zero?
> > > >
> > >
> > > If the allocation order is not high then
> > >
> > > pgdat_needs_compaction == (order > 0) == false == no calling compact_pdatt
> > >
> > > In the case where order is reset to 0 due to fragmentation then it does
> > > call compact_pgdat but it does no work due to the cc->order check in
> > > __compact_pgdat.
> > >
> >
> > I am looking at mmotm-2013-08-07-16-55 but couldn't find cc->order
> > check right before compact_zone in __comact_pgdat.
> > Could you pinpoint code piece?
> >
>
> Thanks, I screwed up as that check happens too late. However, it still
> ends up not mattering because it does this
>
> compact_pgdat
> -> __compact_pgdat
> -> compact_zone
> -> compaction_suitable
>
> For order == 0, compaction_suitable will return either COMPACT_SKIPPED
> (if the watermarks are not met) and COMPACT_PARTIAL otherwise. Either
> way, compaction doesn't run.
In compaction_suitable, it could pass first zone_watermark_ok
but failed second zone_watermark_ok while fragindex is -1000
so compaction could run.
And we shouldn't depend on such coincidence.
>
> --
> Mel Gorman
> SUSE Labs
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to [email protected]. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
--
Kind regards,
Minchan Kim
If kswapd was reclaiming for a high order and resets it to 0 due to
fragmentation it will still call compact_pgdat. For the most part, this will
fail a compaction_suitable() test and not compact but it is unnecessarily
sloppy. It could be fixed in the caller but fix it in the API instead.
[[email protected]: Pointed out that it was a potential problem]
Signed-off-by: Mel Gorman <[email protected]>
---
mm/compaction.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/mm/compaction.c b/mm/compaction.c
index 05ccb4c..c437893 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1131,6 +1131,9 @@ void compact_pgdat(pg_data_t *pgdat, int order)
.sync = false,
};
+ if (!order)
+ return;
+
__compact_pgdat(pgdat, &cc);
}
On Thu, Aug 15, 2013 at 04:39:27PM +0100, Mel Gorman wrote:
> If kswapd was reclaiming for a high order and resets it to 0 due to
> fragmentation it will still call compact_pgdat. For the most part, this will
> fail a compaction_suitable() test and not compact but it is unnecessarily
> sloppy. It could be fixed in the caller but fix it in the API instead.
>
> [[email protected]: Pointed out that it was a potential problem]
> Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Minchan Kim <[email protected]>
--
Kind regards,
Minchan Kim