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Message-ID: <20170620212107-mutt-send-email-mst@kernel.org>
Date: Tue, 20 Jun 2017 21:26:15 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Rik van Riel <riel@...hat.com>
Cc: David Hildenbrand <david@...hat.com>,
Dave Hansen <dave.hansen@...el.com>,
Wei Wang <wei.w.wang@...el.com>, linux-kernel@...r.kernel.org,
qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
kvm@...r.kernel.org, linux-mm@...ck.org, cornelia.huck@...ibm.com,
akpm@...ux-foundation.org, mgorman@...hsingularity.net,
aarcange@...hat.com, amit.shah@...hat.com, pbonzini@...hat.com,
liliang.opensource@...il.com, Nitesh Narayan Lal <nilal@...hat.com>
Subject: Re: [PATCH v11 4/6] mm: function to offer a page block on the free
list
On Tue, Jun 20, 2017 at 01:29:00PM -0400, Rik van Riel wrote:
> On Tue, 2017-06-20 at 18:49 +0200, David Hildenbrand wrote:
> > On 20.06.2017 18:44, Rik van Riel wrote:
>
> > > Nitesh Lal (on the CC list) is working on a way
> > > to efficiently batch recently freed pages for
> > > free page hinting to the hypervisor.
> > >
> > > If that is done efficiently enough (eg. with
> > > MADV_FREE on the hypervisor side for lazy freeing,
> > > and lazy later re-use of the pages), do we still
> > > need the harder to use batch interface from this
> > > patch?
> > >
> >
> > David's opinion incoming:
> >
> > No, I think proper free page hinting would be the optimum solution,
> > if
> > done right. This would avoid the batch interface and even turn
> > virtio-balloon in some sense useless.
>
> I agree with that. Let me go into some more detail of
> what Nitesh is implementing:
>
> 1) In arch_free_page, the being-freed page is added
> to a per-cpu set of freed pages.
> 2) Once that set is full, arch_free_pages goes into a
> slow path, which:
> 2a) Iterates over the set of freed pages, and
> 2b) Checks whether they are still free, and
> 2c) Adds the still free pages to a list that is
> to be passed to the hypervisor, to be MADV_FREEd.
> 2d) Makes that hypercall.
>
> Meanwhile all arch_alloc_pages has to do is make sure it
> does not allocate a page while it is currently being
> MADV_FREEd on the hypervisor side.
>
> The code Wei is working on looks like it could be
> suitable for steps (2c) and (2d) above. Nitesh already
> has code for steps 1 through 2b.
>
> --
> All rights reversed
So my question is this: Wei posted these numbers for balloon
inflation times:
inflating 7GB of an 8GB idle guest:
1) allocating pages (6.5%)
2) sending PFNs to host (68.3%)
3) address translation (6.1%)
4) madvise (19%)
It takes about 4126ms for the inflating process to complete.
It seems that this is an excessive amount of time to stay
under a lock. What are your estimates for Nitesh's work?
--
MST
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