Re: [PATCH 09/22] mm, compaction: Use free lists to quickly locate a migration source

[Vlastimil Babka]

On 1/18/19 6:51 PM, Mel Gorman wrote:
> The migration scanner is a linear scan of a zone with a potentiall large
> search space.  Furthermore, many pageblocks are unusable such as those
> filled with reserved pages or partially filled with pages that cannot
> migrate. These still get scanned in the common case of allocating a THP
> and the cost accumulates.
> 
> The patch uses a partial search of the free lists to locate a migration
> source candidate that is marked as MOVABLE when allocating a THP. It
> prefers picking a block with a larger number of free pages already on
> the basis that there are fewer pages to migrate to free the entire block.
> The lowest PFN found during searches is tracked as the basis of the start
> for the linear search after the first search of the free list fails.
> After the search, the free list is shuffled so that the next search will
> not encounter the same page. If the search fails then the subsequent
> searches will be shorter and the linear scanner is used.
> 
> If this search fails, or if the request is for a small or
> unmovable/reclaimable allocation then the linear scanner is still used. It
> is somewhat pointless to use the list search in those cases. Small free
> pages must be used for the search and there is no guarantee that movable
> pages are located within that block that are contiguous.
> 
>                                      5.0.0-rc1              5.0.0-rc1
>                                  noboost-v3r10          findmig-v3r15
> Amean     fault-both-3      3771.41 (   0.00%)     3390.40 (  10.10%)
> Amean     fault-both-5      5409.05 (   0.00%)     5082.28 (   6.04%)
> Amean     fault-both-7      7040.74 (   0.00%)     7012.51 (   0.40%)
> Amean     fault-both-12    11887.35 (   0.00%)    11346.63 (   4.55%)
> Amean     fault-both-18    16718.19 (   0.00%)    15324.19 (   8.34%)
> Amean     fault-both-24    21157.19 (   0.00%)    16088.50 *  23.96%*
> Amean     fault-both-30    21175.92 (   0.00%)    18723.42 *  11.58%*
> Amean     fault-both-32    21339.03 (   0.00%)    18612.01 *  12.78%*
> 
>                                 5.0.0-rc1              5.0.0-rc1
>                             noboost-v3r10          findmig-v3r15
> Percentage huge-3        86.50 (   0.00%)       89.83 (   3.85%)
> Percentage huge-5        92.52 (   0.00%)       91.96 (  -0.61%)
> Percentage huge-7        92.44 (   0.00%)       92.85 (   0.44%)
> Percentage huge-12       92.98 (   0.00%)       92.74 (  -0.25%)
> Percentage huge-18       91.70 (   0.00%)       91.71 (   0.02%)
> Percentage huge-24       91.59 (   0.00%)       92.13 (   0.60%)
> Percentage huge-30       90.14 (   0.00%)       93.79 (   4.04%)
> Percentage huge-32       90.03 (   0.00%)       91.27 (   1.37%)
> 
> This shows an improvement in allocation latencies with similar allocation
> success rates.  While not presented, there was a 31% reduction in migration
> scanning and a 8% reduction on system CPU usage. A 2-socket machine showed
> similar benefits.
> 
> [vbabka@xxxxxxx: Migrate block that was found-fast, some optimisations]
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

With the followup fix,

Acked-by: Vlastimil Babka <Vbabka@xxxxxxx>