Re: [PATCH 05/14] mm, compaction: Skip pageblocks with reserved pages

Vlastimil Babka
2018-12-18T08:08:07Z

On 12/15/18 12:03 AM, Mel Gorman wrote:
> Reserved pages are set at boot time, tend to be clustered and almost
> never become unreserved. When isolating pages for migrating, skip
> the entire pageblock is one PageReserved page is encountered on the
> grounds that it is highly probable the entire pageblock is reserved.

Agreed, but maybe since it's highly probable and not certain, this
skipping should not be done on the highest compaction priority?

> The impact depends on the machine and timing but both thpscale and
> thpfioscale when using MADV_HUGEPAGE show a reduction of scanning and
> fault latency on a 1-socket machine. The 2-socket results were too
> noisy to draw any meaningful conclusion but it's safe to assume less
> scanning is useful.
>
> 1-socket thpfioscale
> 4.20.0-rc6 4.20.0-rc6
> mmotm-20181210 noreserved-v1r4
> Amean fault-base-1 1481.32 ( 0.00%) 1443.63 ( 2.54%)
> Amean fault-huge-1 1118.17 ( 0.00%) 981.30 * 12.24%*
> Amean fault-both-1 1176.43 ( 0.00%) 1052.64 * 10.52%*
>
> Compaction migrate scanned 3860713 3294284
> Compaction free scanned 613786341 433423502
> Kcompactd migrate scanned 408711 291915
> Kcompactd free scanned 242509759 217164988
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> mm/compaction.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 3afa4e9188b6..8134dba47584 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -827,6 +827,13 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> goto isolate_success;
> }
>
> + /*
> + * A reserved page is never freed and tend to be
> + * clustered in the same pageblocks. Skip the block.
> + */
> + if (PageReserved(page))
> + low_pfn = end_pfn;
> +
> goto isolate_fail;
> }
>
>