Re: [PATCH 4/5] mm: Reclaim small amounts of memory when an external fragmentation event occurs

[Vlastimil Babka]

On 11/23/18 12:45 PM, Mel Gorman wrote:
> An external fragmentation event was previously described as
> 
>     When the page allocator fragments memory, it records the event using
>     the mm_page_alloc_extfrag event. If the fallback_order is smaller
>     than a pageblock order (order-9 on 64-bit x86) then it's considered
>     an event that will cause external fragmentation issues in the future.
> 
> The kernel reduces the probability of such events by increasing the
> watermark sizes by calling set_recommended_min_free_kbytes early in the
> lifetime of the system. This works reasonably well in general but if there
> are enough sparsely populated pageblocks then the problem can still occur
> as enough memory is free overall and kswapd stays asleep.
> 
> This patch introduces a watermark_boost_factor sysctl that allows a zone
> watermark to be temporarily boosted when an external fragmentation causing
> events occurs. The boosting will stall allocations that would decrease
> free memory below the boosted low watermark and kswapd is woken if the
> calling context allows to reclaim an amount of memory relative to the
> size of the high watermark and the watermark_boost_factor until the boost
> is cleared. When kswapd finishes, it wakes kcompactd at the pageblock
> order to clean some of the pageblocks that may have been affected by
> the fragmentation event. kswapd avoids any writeback, slab shrinkage and
> swap from reclaim context during this operation to avoid excessive system
> disruption in the name of fragmentation avoidance. Care is taken so that
> kswapd will do normal reclaim work if the system is really low on memory.
> 
> This was evaluated using the same workloads as "mm, page_alloc: Spread
> allocations across zones before introducing fragmentation".
> 
> 1-socket Skylake machine
> config-global-dhp__workload_thpfioscale XFS (no special madvise)
> 4 fio threads, 1 THP allocating thread
> --------------------------------------
> 
> 4.20-rc3 extfrag events < order 9:   804694
> 4.20-rc3+patch:                      408912 (49% reduction)
> 4.20-rc3+patch1-4:                    18421 (98% reduction)
> 
>                                    4.20.0-rc3             4.20.0-rc3
>                                  lowzone-v5r8             boost-v5r8
> Amean     fault-base-1      653.58 (   0.00%)      652.71 (   0.13%)
> Amean     fault-huge-1        0.00 (   0.00%)      178.93 * -99.00%*
> 
>                               4.20.0-rc3             4.20.0-rc3
>                             lowzone-v5r8             boost-v5r8
> Percentage huge-1        0.00 (   0.00%)        5.12 ( 100.00%)
> 
> Note that external fragmentation causing events are massively reduced
> by this path whether in comparison to the previous kernel or the vanilla
> kernel. The fault latency for huge pages appears to be increased but that
> is only because THP allocations were successful with the patch applied.
> 
> 1-socket Skylake machine
> global-dhp__workload_thpfioscale-madvhugepage-xfs (MADV_HUGEPAGE)
> -----------------------------------------------------------------
> 
> 4.20-rc3 extfrag events < order 9:  291392
> 4.20-rc3+patch:                     191187 (34% reduction)
> 4.20-rc3+patch1-4:                   13464 (95% reduction)
> 
> thpfioscale Fault Latencies
>                                    4.20.0-rc3             4.20.0-rc3
>                                  lowzone-v5r8             boost-v5r8
> Min       fault-base-1      912.00 (   0.00%)      905.00 (   0.77%)
> Min       fault-huge-1      127.00 (   0.00%)      135.00 (  -6.30%)
> Amean     fault-base-1     1467.55 (   0.00%)     1481.67 (  -0.96%)
> Amean     fault-huge-1     1127.11 (   0.00%)     1063.88 *   5.61%*
> 
>                               4.20.0-rc3             4.20.0-rc3
>                             lowzone-v5r8             boost-v5r8
> Percentage huge-1       77.64 (   0.00%)       83.46 (   7.49%)
> 
> As before, massive reduction in external fragmentation events, some jitter
> on latencies and an increase in THP allocation success rates.
> 
> 2-socket Haswell machine
> config-global-dhp__workload_thpfioscale XFS (no special madvise)
> 4 fio threads, 5 THP allocating threads
> ----------------------------------------------------------------
> 
> 4.20-rc3 extfrag events < order 9:  215698
> 4.20-rc3+patch:                     200210 (7% reduction)
> 4.20-rc3+patch1-4:                   14263 (93% reduction)
> 
>                                    4.20.0-rc3             4.20.0-rc3
>                                  lowzone-v5r8             boost-v5r8
> Amean     fault-base-5     1346.45 (   0.00%)     1306.87 (   2.94%)
> Amean     fault-huge-5     3418.60 (   0.00%)     1348.94 (  60.54%)
> 
>                               4.20.0-rc3             4.20.0-rc3
>                             lowzone-v5r8             boost-v5r8
> Percentage huge-5        0.78 (   0.00%)        7.91 ( 910.64%)
> 
> There is a 93% reduction in fragmentation causing events, there
> is a big reduction in the huge page fault latency and allocation
> success rate is higher.
> 
> 2-socket Haswell machine
> global-dhp__workload_thpfioscale-madvhugepage-xfs (MADV_HUGEPAGE)
> -----------------------------------------------------------------
> 
> 4.20-rc3 extfrag events < order 9: 166352
> 4.20-rc3+patch:                    147463 (11% reduction)
> 4.20-rc3+patch1-4:                  11095 (93% reduction)
> 
> thpfioscale Fault Latencies
>                                    4.20.0-rc3             4.20.0-rc3
>                                  lowzone-v5r8             boost-v5r8
> Amean     fault-base-5     6217.43 (   0.00%)     7419.67 * -19.34%*
> Amean     fault-huge-5     3163.33 (   0.00%)     3263.80 (  -3.18%)
> 
>                               4.20.0-rc3             4.20.0-rc3
>                             lowzone-v5r8             boost-v5r8
> Percentage huge-5       95.14 (   0.00%)       87.98 (  -7.53%)
> 
> There is a large reduction in fragmentation events with some jitter around
> the latencies and success rates. As before, the high THP allocation
> success rate does mean the system is under a lot of pressure. However,
> as the fragmentation events are reduced, it would be expected that the
> long-term allocation success rate would be higher.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>