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    SubjectRe: [PATCH 16/22] Do not setup zonelist cache when there is only one node
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    > On Wed, 22 Apr 2009, Lee Schermerhorn wrote:
    >
    > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > > > > index 7f45de1..e59bb80 100644
    > > > > --- a/mm/page_alloc.c
    > > > > +++ b/mm/page_alloc.c
    > > > > @@ -1467,8 +1467,11 @@ this_zone_full:
    > > > > if (NUMA_BUILD)
    > > > > zlc_mark_zone_full(zonelist, z);
    > > >
    > > > If zonelist caching is never used for UMA machines, why should they ever
    > > > call zlc_mark_zone_full()? It will always dereference
    > > > zonelist->zlcache_ptr and immediately return without doing anything.
    > > >
    > > > Wouldn't it better to just add
    > > >
    > > > if (num_online_nodes() == 1)
    > > > continue;
    > > >
    > > > right before this call to zlc_mark_zone_full()? This should compile out
    > > > the remainder of the loop for !CONFIG_NUMA kernels anyway.
    > >
    > > Shouldn't it already do that? NUMA_BUILD is defined as 0 when
    > > !CONFIG_NUMA to avoid #ifdef's in the code while still allowing compiler
    > > error checking in the dead code.
    > >
    >
    > Yeah, but adding the check on num_online_nodes() also prevents needlessly
    > calling zlc_mark_zone_full() on CONFIG_NUMA kernels when running on an UMA
    > machine.

    I don't like this idea...

    In UMA system, zlc_mark_zone_full() isn't so expensive. but In large system
    one branch increasing is often costly.





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