Re: [PATCH 3/3] mm: page allocator: Drain per-cpu lists after direct reclaim allocation fails

[KOSAKI Motohiro]

> On Thu, Sep 09, 2010 at 09:32:52AM -0500, Christoph Lameter wrote:
> > On Thu, 9 Sep 2010, Mel Gorman wrote:
> > 
> > > > This will have the effect of never sending IPIs for slab allocations since
> > > > they do not do allocations for orders > PAGE_ALLOC_COSTLY_ORDER.
> > > >
> > >
> > > The question is how severe is that? There is somewhat of an expectation
> > > that the lower orders free naturally so it the IPI justified? That said,
> > > our historical behaviour would have looked like
> > >
> > > if (!page && !drained && order) {
> > > 	drain_all_pages();
> > > 	draiained = true;
> > > 	goto retry;
> > > }
> > >
> > > Play it safe for now and go with that?
> > 
> > I am fine with no IPIs for order <= COSTLY. Just be aware that this is
> > a change that may have some side effects.
> 
> I made the choice consciously. I felt that if slab or slub were depending on
> IPIs to make successful allocations in low-memory conditions that it would
> experience varying stalls on bigger machines due to increased interrupts that
> might be difficult to diagnose while not necessarily improving allocation
> success rates. I also considered that if the machine is under pressure then
> slab and slub may also be releasing pages of the same order and effectively
> recycling their pages without depending on IPIs.

+1.

In these days, average numbers of CPUs are increasing. So we need to be afraid
IPI storm than past.



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