OK, I think I got it. Thank you very much!!


2014/1/12 Johannes Weiner <hannes@cmpxchg.org>

> On Tue, Jan 07, 2014 at 09:37:01AM +0800, yvxiang wrote:
> > Hi, Johannes
> >
> >      I'm a new comer to vm. And I read your commit 81c0a2bb about fair
> zone
> > allocator policy,  but I don't quite understand your opinion, especially
> > the words that
> >
> >    "the allocator may keep kswapd running while kswapd reclaim
> >     ensures that the page allocator can keep allocating from the first
> zone
> > in
> >     the zonelist for extended periods of time. "
> >
> >     Could you or someone else explain me what does this mean in more
> > details? Or could you give me a example?
>
> The page allocator tries to allocate from all zones in order of
> preference: Normal, DMA32, DMA.  If they are all at their low
> watermark, kswapd is woken up and it will reclaim each zone until it's
> back to the high watermark.
>
> But as kswapd reclaims the Normal zone, the page allocator can
> continue allocating from it.  If that happens at roughly the same
> pace, the Normal zone's watermark will hover somewhere between the low
> and high watermark.  Kswapd will not go to sleep and the page
> allocator will not use the other zones.
>
> The whole workload's memory will be allocated and reclaimed using only
> the Normal zone, which might be only a few (hundred) megabytes, while
> the 4G DMA32 zone is unused.
>