Re: [RFC/T/D][PATCH 2/2] Linux/Guest cooperative unmapped page cache control

Balbir Singh
2010-06-11T06:14:54Z

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-06-11 14:05:53]:

> On Fri, 11 Jun 2010 10:16:32 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-06-11 10:54:41]:
> >
> > > On Thu, 10 Jun 2010 17:07:32 -0700
> > > Dave Hansen <dave@linux.vnet.ibm.com> wrote:
> > >
> > > > On Thu, 2010-06-10 at 19:55 +0530, Balbir Singh wrote:
> > > > > > I'm not sure victimizing unmapped cache pages is a good idea.
> > > > > > Shouldn't page selection use the LRU for recency information instead
> > > > > > of the cost of guest reclaim? Dropping a frequently used unmapped
> > > > > > cache page can be more expensive than dropping an unused text page
> > > > > > that was loaded as part of some executable's initialization and
> > > > > > forgotten.
> > > > >
> > > > > We victimize the unmapped cache only if it is unused (in LRU order).
> > > > > We don't force the issue too much. We also have free slab cache to go
> > > > > after.
> > > >
> > > > Just to be clear, let's say we have a mapped page (say of /sbin/init)
> > > > that's been unreferenced since _just_ after the system booted. We also
> > > > have an unmapped page cache page of a file often used at runtime, say
> > > > one from /etc/resolv.conf or /etc/passwd.
> > > >
> > >
> > > Hmm. I'm not fan of estimating working set size by calculation
> > > based on some numbers without considering history or feedback.
> > >
> > > Can't we use some kind of feedback algorithm as hi-low-watermark, random walk
> > > or GA (or somehing more smart) to detect the size ?
> > >
> >
> > Could you please clarify at what level you are suggesting size
> > detection? I assume it is outside the OS, right?
> >
> "OS" includes kernel and system programs ;)
>
> I can think of both way in kernel and in user approarh and they should be
> complement to each other.
>
> An example of kernel-based approach is.
> 1. add a shrinker callback(A) for balloon-driver-for-guest as guest kswapd.
> 2. add a shrinker callback(B) for balloon-driver-for-host as host kswapd.
> (I guess current balloon driver is only for host. Please imagine.)
>
> (A) increases free memory in Guest.
> (B) increases free memory in Host.
>
> This is an example of feedback based memory resizing between host and guest.
>
> I think (B) is necessary at least before considering complecated things.

B is left to the hypervisor and the memory policy running on it. My
patches address Linux running as a guest, with a Linux hypervisor at
the moment, but that can be extended to other balloon drivers as well.

>
> To implement something clever, (A) and (B) should take into account that
> how frequently memory reclaim in guest (which requires some I/O) happens.
>

Yes, I think the policy in the hypervisor needs to look at those
details as well.

> If doing outside kernel, I think using memcg is better than depends on
> balloon driver. But co-operative balloon and memcg may show us something
> good.
>

Yes, agreed. Co-operative is better, if there is no co-operation than
memcg might be used for enforcement.

--
Three Cheers,
Balbir