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Subject: Re: [PATCH 05/14] mm, compaction: Skip pageblocks with reserved pages
To:     Mel Gorman <mgorman@techsingularity.net>,
        Linux-MM <linux-mm@kvack.org>
Cc:     David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>, ying.huang@intel.com,
        kirill@shutemov.name, Andrew Morton <akpm@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
References: <20181214230310.572-1-mgorman@techsingularity.net>
 <20181214230310.572-6-mgorman@techsingularity.net>
From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Tue, 18 Dec 2018 09:08:02 +0100
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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;
>  		}
>  
>