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List:       linux-mm
Subject:    Re: [PATCH 11/22] mm, compaction: Use free lists to quickly locate a migration target
From:       Vlastimil Babka <vbabka () suse ! cz>
Date:       2019-01-31 14:52:10
Message-ID: 81e45dc0-c107-015b-e167-19d7ca4b6374 () suse ! cz
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On 1/18/19 6:51 PM, Mel Gorman wrote:
> Similar to the migration scanner, this patch uses the free lists to quickly
> locate a migration target. The search is different in that lower orders
> will be searched for a suitable high PFN if necessary but the search
> is still bound. This is justified on the grounds that the free scanner
> typically scans linearly much more than the migration scanner.
> 
> If a free page is found, it is isolated and compaction continues if enough
> pages were isolated. For SYNC* scanning, the full pageblock is scanned
> for any remaining free pages so that is can be marked for skipping in
> the near future.
> 
> 1-socket thpfioscale
>                                      5.0.0-rc1              5.0.0-rc1
>                                  isolmig-v3r15         findfree-v3r16
> Amean     fault-both-3      3024.41 (   0.00%)     3200.68 (  -5.83%)
> Amean     fault-both-5      4749.30 (   0.00%)     4847.75 (  -2.07%)
> Amean     fault-both-7      6454.95 (   0.00%)     6658.92 (  -3.16%)
> Amean     fault-both-12    10324.83 (   0.00%)    11077.62 (  -7.29%)
> Amean     fault-both-18    12896.82 (   0.00%)    12403.97 (   3.82%)
> Amean     fault-both-24    13470.60 (   0.00%)    15607.10 * -15.86%*
> Amean     fault-both-30    17143.99 (   0.00%)    18752.27 (  -9.38%)
> Amean     fault-both-32    17743.91 (   0.00%)    21207.54 * -19.52%*
> 
> The impact on latency is variable but the search is optimistic and
> sensitive to the exact system state. Success rates are similar but
> the major impact is to the rate of scanning
> 
>                                 5.0.0-rc1      5.0.0-rc1
>                             isolmig-v3r15 findfree-v3r16
> Compaction migrate scanned    25646769          29507205
> Compaction free scanned      201558184         100359571
> 
> The free scan rates are reduced by 50%. The 2-socket reductions for the
> free scanner are more dramatic which is a likely reflection that the
> machine has more memory.
> 
> [dan.carpenter@oracle.com: Fix static checker warning]
> [vbabka@suse.cz: Correct number of pages scanned for lower orders]
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

Small fix below:

> -/* Reorder the free list to reduce repeated future searches */
> +/*
> + * Used when scanning for a suitable migration target which scans freelists
> + * in reverse. Reorders the list such as the unscanned pages are scanned
> + * first on the next iteration of the free scanner
> + */
> +static void
> +move_freelist_head(struct list_head *freelist, struct page *freepage)
> +{
> +	LIST_HEAD(sublist);
> +
> +	if (!list_is_last(freelist, &freepage->lru)) {

Shouldn't there be list_is_first() for symmetry?

> +		list_cut_before(&sublist, freelist, &freepage->lru);
> +		if (!list_empty(&sublist))
> +			list_splice_tail(&sublist, freelist);
> +	}
> +}
> +
> +/*
> + * Similar to move_freelist_head except used by the migration scanner
> + * when scanning forward. It's possible for these list operations to
> + * move against each other if they search the free list exactly in
> + * lockstep.
> + */
>  static void
>  move_freelist_tail(struct list_head *freelist, struct page *freepage)
>  {

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