Re: [PATCH 3/3] Provide control over unmapped pages

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-12-01 14:35:50]:

> On Wed, 1 Dec 2010 10:52:59 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
> > * Balbir Singh <balbir@linux.vnet.ibm.com> [2010-12-01 10:48:16]:
> > 
> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-12-01 10:32:54]:
> > > 
> > > > On Tue, 30 Nov 2010 15:46:31 +0530
> > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > > > 
> > > > > Provide control using zone_reclaim() and a boot parameter. The
> > > > > code reuses functionality from zone_reclaim() to isolate unmapped
> > > > > pages and reclaim them as a priority, ahead of other mapped pages.
> > > > > 
> > > > > Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
> > > > > ---
> > > > >  include/linux/swap.h |    5 ++-
> > > > >  mm/page_alloc.c      |    7 +++--
> > > > >  mm/vmscan.c          |   72 +++++++++++++++++++++++++++++++++++++++++++++++++-
> > > > >  3 files changed, 79 insertions(+), 5 deletions(-)
> > > > > 
> > > > > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > > > > index eba53e7..78b0830 100644
> > > > > --- a/include/linux/swap.h
> > > > > +++ b/include/linux/swap.h
> > > > > @@ -252,11 +252,12 @@ extern int vm_swappiness;
> > > > >  extern int remove_mapping(struct address_space *mapping, struct page *page);
> > > > >  extern long vm_total_pages;
> > > > >  
> > > > > -#ifdef CONFIG_NUMA
> > > > > -extern int zone_reclaim_mode;
> > > > >  extern int sysctl_min_unmapped_ratio;
> > > > >  extern int sysctl_min_slab_ratio;
> > > > >  extern int zone_reclaim(struct zone *, gfp_t, unsigned int);
> > > > > +extern bool should_balance_unmapped_pages(struct zone *zone);
> > > > > +#ifdef CONFIG_NUMA
> > > > > +extern int zone_reclaim_mode;
> > > > >  #else
> > > > >  #define zone_reclaim_mode 0
> > > > >  static inline int zone_reclaim(struct zone *z, gfp_t mask, unsigned int order)
> > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > > index 62b7280..4228da3 100644
> > > > > --- a/mm/page_alloc.c
> > > > > +++ b/mm/page_alloc.c
> > > > > @@ -1662,6 +1662,9 @@ zonelist_scan:
> > > > >  			unsigned long mark;
> > > > >  			int ret;
> > > > >  
> > > > > +			if (should_balance_unmapped_pages(zone))
> > > > > +				wakeup_kswapd(zone, order);
> > > > > +
> > > > 
> > > > Hm, I'm not sure the final vision of this feature. Does this reclaiming feature
> > > > can't be called directly via balloon driver just before alloc_page() ?
> > > >
> > > 
> > > That is a separate patch, this is a boot paramter based control
> > > approach.
> > >  
> > > > Do you need to keep page caches small even when there are free memory on host ?
> > > >
> > > 
> > > The goal is to avoid duplication, as you know page cache fills itself
> > > to consume as much memory as possible. The host generally does not
> > > have a lot of free memory in a consolidated environment. 
> > >
> 
> That's a point. Then, why the guest has to do _extra_ work for host even when
> the host says nothing ? I think trigger this by guests themselves is not very good.

I've mentioned it before, the guest keeping free memory without a
large performance hit, helps, the balloon driver is able to quickly
retrieve this memory if required or the guest can use this memory for
some other application/task. The cached data is mostly already present
in the host page cache.

-- 
	Three Cheers,
	Balbir

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