Re: [RFC][PATCH 1/2] Linux/Guest unmapped page cache control

[Balbir Singh]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-06-14 09:28:19]:

> On Mon, 14 Jun 2010 00:01:45 +0530
> Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > * Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> [2010-06-08 21:21:46]:
> > 
> > > Selectively control Unmapped Page Cache (nospam version)
> > > 
> > > From: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> > > 
> > > This patch implements unmapped page cache control via preferred
> > > page cache reclaim. The current patch hooks into kswapd and reclaims
> > > page cache if the user has requested for unmapped page control.
> > > This is useful in the following scenario
> > > 
> > > - In a virtualized environment with cache=writethrough, we see
> > >   double caching - (one in the host and one in the guest). As
> > >   we try to scale guests, cache usage across the system grows.
> > >   The goal of this patch is to reclaim page cache when Linux is running
> > >   as a guest and get the host to hold the page cache and manage it.
> > >   There might be temporary duplication, but in the long run, memory
> > >   in the guests would be used for mapped pages.
> > > - The option is controlled via a boot option and the administrator
> > >   can selectively turn it on, on a need to use basis.
> > > 
> > > A lot of the code is borrowed from zone_reclaim_mode logic for
> > > __zone_reclaim(). One might argue that the with ballooning and
> > > KSM this feature is not very useful, but even with ballooning,
> > > we need extra logic to balloon multiple VM machines and it is hard
> > > to figure out the correct amount of memory to balloon. With these
> > > patches applied, each guest has a sufficient amount of free memory
> > > available, that can be easily seen and reclaimed by the balloon driver.
> > > The additional memory in the guest can be reused for additional
> > > applications or used to start additional guests/balance memory in
> > > the host.
> > > 
> > > KSM currently does not de-duplicate host and guest page cache. The goal
> > > of this patch is to help automatically balance unmapped page cache when
> > > instructed to do so.
> > > 
> > > There are some magic numbers in use in the code, UNMAPPED_PAGE_RATIO
> > > and the number of pages to reclaim when unmapped_page_control argument
> > > is supplied. These numbers were chosen to avoid aggressiveness in
> > > reaping page cache ever so frequently, at the same time providing control.
> > > 
> > > The sysctl for min_unmapped_ratio provides further control from
> > > within the guest on the amount of unmapped pages to reclaim.
> > >
> > 
> > Are there any major objections to this patch?
> >  
> 
> This kind of patch needs "how it works well" measurement.
> 
> - How did you measure the effect of the patch ? kernbench is not enough, of course.

I can run other benchmarks as well, I will do so

> - Why don't you believe LRU ? And if LRU doesn't work well, should it be
>   fixed by a knob rather than generic approach ?
> - No side effects ?

I believe in LRU, just that the problem I am trying to solve is of
using double the memory for caching the same data (consider kvm
running in cache=writethrough or writeback mode, both the hypervisor
and the guest OS maintain a page cache of the same data). As the VM's
grow the overhead is substantial. In my runs I found upto 60%
duplication in some cases.

> 
> - Linux vm guys tend to say, "free memory is bad memory". ok, for what
>   free memory created by your patch is used ? IOW, I can't see the benefit.
>   If free memory that your patch created will be used for another page-cache,
>   it will be dropped soon by your patch itself.
> 

Free memory is good for cases when you want to do more in the same
system. I agree that in a bare metail environment that might be
partially true. I don't have a problem with frequently used data being
cached, but I am targetting a consolidated environment at the moment.
Moreover, the administrator has control via a boot option, so it is
non-instrusive in many ways.

>   If your patch just drops "duplicated, but no more necessary for other kvm",
>   I agree your patch may increase available size of page-caches. But you just
>   drops unmapped pages.
>

unmapped and unused are the best targets, I plan to add slab cache control later. 

-- 
	Three Cheers,
	Balbir
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