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SubjectRe: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression
On Mon, 3 Dec 2018, Andrea Arcangeli wrote:

> In my earlier review of David's patch, it looked runtime equivalent to
> the __GFP_COMPACT_ONLY solution. It has the only advantage of adding a
> new gfpflag until we're sure we need it but it's the worst solution
> available for the long term in my view. It'd be ok to apply it as
> stop-gap measure though.
>
> The "order == pageblock_order" hardcoding inside the allocator to
> workaround the __GFP_THISNODE flag passed from outside the allocator
> in the THP MADV_HUGEPAGE case, didn't look very attractive because
> it's not just THP allocating order >0 pages.
>

We have two different things to consider: NUMA locality and the order of
the allocation. THP is preferred locally and we know the order. For the
other high-order pages you're referring to, I don't know if they are using
__GFP_THISNODE or not (likely not). I see them as two separate issues.

For thp on all platforms I have measured it on specifically for this patch
(Broadwell, Haswell, Rome) there is a clear advantage to faulting local
pages of the native page size over remote hugepages. It also has the
added effect of allowing khugepaged to collapse it into a hugepage later
if fragmentation allows (the reason why khugepaged cares about NUMA
locality, the same reason I do). This is the rationale for __GFP_THISNODE
for thp allocations.

For order == pageblock_order (or more correctly order >= pageblock_order),
this is not based on NUMA whatsoever but is rather based on the
implementation of memory compaction. If it has already failed (or was
deferred for order-HPAGE_PMD_ORDER), reclaim cannot be shown to help if
memory compaction cannot utilize the freed memory in isolate_freepages(),
so that reclaim has been pointless. If compaction fails for other reasons
(any unmovable page preventing a pageblock from becoming free), *all*
reclaim activity has been pointless.

> It'd be nicer if whatever compaction latency optimization that applies
> to THP could also apply to all other allocation orders too and the
> hardcoding of the THP order prevents that.
>
> On the same lines if __GFP_THISNODE is so badly needed by
> MADV_HUGEPAGE, all other larger order allocations should also be able
> to take advantage of __GFP_THISNODE without ending in the same VM
> corner cases that required the "order == pageblock_order" hardcoding
> inside the allocator.
>
> If you prefer David's patch I would suggest pageblock_order to be
> replaced with HPAGE_PMD_ORDER so it's more likely to match the THP
> order in all archs.
>

That sounds fine and I will do that in my v2.

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