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    SubjectRe: [RFC][PATCH 1/2] Linux/Guest unmapped page cache control
    * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-06-14 09:28:19]:

    > On Mon, 14 Jun 2010 00:01:45 +0530
    > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
    >
    > > * Balbir Singh <balbir@linux.vnet.ibm.com> [2010-06-08 21:21:46]:
    > >
    > > > Selectively control Unmapped Page Cache (nospam version)
    > > >
    > > > From: Balbir Singh <balbir@linux.vnet.ibm.com>
    > > >
    > > > 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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