Re: [patch] mm: memcontrol: support transparent huge pages under pressure

From: Vladimir Davydov
Date: Mon Sep 22 2014 - 06:18:17 EST


On Fri, Sep 19, 2014 at 09:20:40AM -0400, Johannes Weiner wrote:
> In a memcg with even just moderate cache pressure, success rates for
> transparent huge page allocations drop to zero, wasting a lot of
> effort that the allocator puts into assembling these pages.
>
> The reason for this is that the memcg reclaim code was never designed
> for higher-order charges. It reclaims in small batches until there is
> room for at least one page. Huge pages charges only succeed when
> these batches add up over a series of huge faults, which is unlikely
> under any significant load involving order-0 allocations in the group.
>
> Remove that loop on the memcg side in favor of passing the actual
> reclaim goal to direct reclaim, which is already set up and optimized
> to meet higher-order goals efficiently.
>
> This brings memcg's THP policy in line with the system policy: if the
> allocator painstakingly assembles a hugepage, memcg will at least make
> an honest effort to charge it. As a result, transparent hugepage
> allocation rates amid cache activity are drastically improved:
>
> vanilla patched
> pgalloc 4717530.80 ( +0.00%) 4451376.40 ( -5.64%)
> pgfault 491370.60 ( +0.00%) 225477.40 ( -54.11%)
> pgmajfault 2.00 ( +0.00%) 1.80 ( -6.67%)
> thp_fault_alloc 0.00 ( +0.00%) 531.60 (+100.00%)
> thp_fault_fallback 749.00 ( +0.00%) 217.40 ( -70.88%)
>
> [ Note: this may in turn increase memory consumption from internal
> fragmentation, which is an inherent risk of transparent hugepages.
> Some setups may have to adjust the memcg limits accordingly to
> accomodate this - or, if the machine is already packed to capacity,
> disable the transparent huge page feature. ]
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Looks like a really nice change to me. FWIW,

Reviewed-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
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