linux-kernel - Re: [PATCH RFC] virtio_balloon: conservative balloon page shrinking

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Message-ID: <51306985-90ac-6e6b-d085-4e076698c48c@i-love.sakura.ne.jp>
Date:   Tue, 11 Feb 2020 23:18:56 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     "Wang, Wei W" <wei.w.wang@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "tysand@...gle.com" <tysand@...gle.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "david@...hat.com" <david@...hat.com>,
        "alexander.h.duyck@...ux.intel.com" 
        <alexander.h.duyck@...ux.intel.com>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "namit@...are.com" <namit@...are.com>
Subject: Re: [PATCH RFC] virtio_balloon: conservative balloon page shrinking

On 2020/02/10 16:27, Wang, Wei W wrote:
>> Well, my comment is rather: "Do not try to reserve guest's memory. In other
>> words, do not try to maintain balloons on the guest side. Since host would
>> be able to cache file data on the host's cache, guests would be able to
>> quickly fetch file data from host's cache via normal I/O requests." ;-)
> 
> Didn't this one. The discussion was about guest pagecache pages v.s. guest balloon pages.
> Why is host's pagecache here?

I'm expecting a mode: "Guests should try to minimize pagecache pages (and teach
host to treat reclaimed pages as if POSIX_FADV_DONTNEED) instead of managing
guest balloon pages". In other words, as if

  while :; sleep 5; echo 1 > /proc/sys/vm/drop_caches; done

is running in the guest's kernel. And as if

  echo 2 > /proc/sys/vm/drop_caches

is triggered in the guest's kernel when host requested guests to reclaim
memory. No long-life balloons. Guest balloons do not need to care about
NUMA. Just leave the management of pagecache pages to the host.