From: Alexander Duyck <alexander.duyck@gmail.com>
To: David Hildenbrand <david@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
Nitesh Narayan Lal <nitesh@redhat.com>,
kvm list <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
Paolo Bonzini <pbonzini@redhat.com>,
lcapitulino@redhat.com, pagupta@redhat.com, wei.w.wang@intel.com,
Yang Zhang <yang.zhang.wz@gmail.com>,
Rik van Riel <riel@surriel.com>,
dodgen@google.com, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
dhildenb@redhat.com, Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting
Date: Wed, 6 Mar 2019 11:24:26 -0800 [thread overview]
Message-ID: <CAKgT0UePn86cnjzietzuqdosjJH3McH2xDQ3ocjbujMKdsk7Pw@mail.gmail.com> (raw)
In-Reply-To: <7b98b7b3-68f5-e4e0-1454-2217f41e46ad@redhat.com>
On Wed, Mar 6, 2019 at 11:18 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 06.03.19 20:08, Alexander Duyck wrote:
> > On Wed, Mar 6, 2019 at 11:00 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 06.03.19 19:43, Michael S. Tsirkin wrote:
> >>> On Wed, Mar 06, 2019 at 01:30:14PM -0500, Nitesh Narayan Lal wrote:
> >>>>>> Here are the results:
> >>>>>>
> >>>>>> Procedure: 3 Guests of size 5GB is launched on a single NUMA node with
> >>>>>> total memory of 15GB and no swap. In each of the guest, memhog is run
> >>>>>> with 5GB. Post-execution of memhog, Host memory usage is monitored by
> >>>>>> using Free command.
> >>>>>>
> >>>>>> Without Hinting:
> >>>>>> Time of execution Host used memory
> >>>>>> Guest 1: 45 seconds 5.4 GB
> >>>>>> Guest 2: 45 seconds 10 GB
> >>>>>> Guest 3: 1 minute 15 GB
> >>>>>>
> >>>>>> With Hinting:
> >>>>>> Time of execution Host used memory
> >>>>>> Guest 1: 49 seconds 2.4 GB
> >>>>>> Guest 2: 40 seconds 4.3 GB
> >>>>>> Guest 3: 50 seconds 6.3 GB
> >>>>> OK so no improvement.
> >>>> If we are looking in terms of memory we are getting back from the guest,
> >>>> then there is an improvement. However, if we are looking at the
> >>>> improvement in terms of time of execution of memhog then yes there is none.
> >>>
> >>> Yes but the way I see it you can't overcommit this unused memory
> >>> since guests can start using it at any time. You timed it carefully
> >>> such that this does not happen, but what will cause this timing on real
> >>> guests?
> >>
> >> Whenever you overcommit you will need backup swap. There is no way
> >> around it. It just makes the probability of you having to go to disk
> >> less likely.
> >>
> >> If you assume that all of your guests will be using all of their memory
> >> all the time, you don't have to think about overcommiting memory in the
> >> first place. But this is not what we usually have.
> >
> > Right, but the general idea is that free page hinting allows us to
> > avoid having to use the swap if we are hinting the pages as unused.
> > The general assumption we are working with is that some percentage of
> > the VMs are unused most of the time so you can share those resources
> > between multiple VMs and have them free those up normally.
>
> Yes, similar to VCPU yielding or playin scheduling when the VCPU is
> spleeping. Instead of busy looping, hand over the resource to somebody
> who can actually make use of it.
>
> >
> > If we can reduce swap usage we can improve overall performance and
> > that was what I was pointing out with my test. I had also done
> > something similar to what Nitesh was doing with his original test
> > where I had launched 8 VMs with 8GB of memory per VM on a system with
> > 32G of RAM and only 4G of swap. In that setup I could keep a couple
> > VMs busy at a time without issues, and obviously without the patch I
> > just started to OOM qemu instances and could only have 4 VMs at a
> > time running at maximum.
>
> While these are nice experiments (especially to showcase reduced swap
> usage!), I would not suggest to use 4GB of swap on a x2 overcomited
> system (32GB overcommited). Disks are so cheap nowadays that one does
> not have to play with fire.
Right. The only reason for using 4G is because the system normally has
128G of RAM available and I didn't really think I would need swap for
the system when I originally configured it.
> But yes, reducing swap usage implies overall system performance (unless
> the hinting is terribly slow :) ). Reducing swap usage, not swap space :)
Right. Also the swap is really a necessity if we are going to look at
things like MADV_FREE as I have not seen us really start to free up
resources until we are starting to put some pressure on swap.
next prev parent reply other threads:[~2019-03-06 19:24 UTC|newest]
Thread overview: 84+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-06 15:50 [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting Nitesh Narayan Lal
2019-03-06 15:50 ` [RFC][Patch v9 1/6] KVM: Guest free page hinting support Nitesh Narayan Lal
2019-03-06 23:43 ` Alexander Duyck
2019-03-07 19:32 ` Nitesh Narayan Lal
2019-03-06 15:50 ` [RFC][Patch v9 2/6] KVM: Enables the kernel to isolate guest free pages Nitesh Narayan Lal
2019-03-07 18:30 ` Alexander Duyck
2019-03-07 19:23 ` Nitesh Narayan Lal
2019-03-07 19:30 ` David Hildenbrand
2019-03-07 21:32 ` Alexander Duyck
2019-03-07 21:40 ` David Hildenbrand
2019-03-07 22:35 ` Alexander Duyck
2019-03-08 2:28 ` Michael S. Tsirkin
2019-03-08 2:32 ` Michael S. Tsirkin
2019-03-08 18:06 ` Alexander Duyck
2019-03-08 18:59 ` Michael S. Tsirkin
2019-03-08 19:10 ` Nitesh Narayan Lal
2019-03-08 19:25 ` Alexander Duyck
2019-03-08 19:38 ` Nitesh Narayan Lal
2019-03-08 21:39 ` Alexander Duyck
2019-03-12 19:46 ` Nitesh Narayan Lal
2019-03-12 21:13 ` Alexander Duyck
2019-03-12 21:53 ` David Hildenbrand
2019-03-12 22:56 ` Alexander Duyck
2019-03-13 11:54 ` Nitesh Narayan Lal
2019-03-13 12:17 ` David Hildenbrand
2019-03-13 13:08 ` Nitesh Narayan Lal
2019-03-13 16:37 ` Alexander Duyck
2019-03-13 16:39 ` David Hildenbrand
2019-03-13 22:54 ` Alexander Duyck
2019-03-13 23:18 ` David Hildenbrand
2019-03-06 15:50 ` [RFC][Patch v9 3/6] KVM: Enables the kernel to report isolated pages Nitesh Narayan Lal
2019-03-06 21:30 ` Alexander Duyck
2019-03-07 13:23 ` Nitesh Narayan Lal
2019-03-06 15:50 ` [RFC][Patch v9 4/6] KVM: Reporting page poisoning value to the host Nitesh Narayan Lal
2019-03-06 15:50 ` [RFC][Patch v9 5/6] KVM: Enabling guest free page hinting via static key Nitesh Narayan Lal
2019-03-06 15:50 ` [RFC][Patch v9 6/6] KVM: Adding tracepoints for guest free page hinting Nitesh Narayan Lal
2019-03-06 15:52 ` [RFC][QEMU Patch] KVM: Enable QEMU to free the pages hinted by the guest Nitesh Narayan Lal
2019-03-06 23:49 ` Alexander Duyck
2019-03-07 0:35 ` Alexander Duyck
2019-03-07 12:23 ` Nitesh Narayan Lal
2019-03-06 16:09 ` [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting Michael S. Tsirkin
2019-03-06 18:07 ` Nitesh Narayan Lal
2019-03-06 18:12 ` Michael S. Tsirkin
2019-03-06 18:30 ` Nitesh Narayan Lal
2019-03-06 18:38 ` Michael S. Tsirkin
2019-03-06 18:40 ` Nitesh Narayan Lal
2019-03-06 18:43 ` Alexander Duyck
2019-03-06 18:43 ` Michael S. Tsirkin
2019-03-06 18:59 ` David Hildenbrand
2019-03-06 19:08 ` Alexander Duyck
2019-03-06 19:18 ` David Hildenbrand
2019-03-06 19:24 ` Alexander Duyck [this message]
2019-03-06 20:31 ` Nitesh Narayan Lal
2019-03-06 20:32 ` Michael S. Tsirkin
2019-03-06 21:40 ` David Hildenbrand
2019-03-06 22:18 ` Michael S. Tsirkin
2019-03-06 23:12 ` Alexander Duyck
2019-03-14 16:42 ` Nitesh Narayan Lal
2019-03-14 16:58 ` Alexander Duyck
2019-03-18 15:57 ` Nitesh Narayan Lal
2019-03-19 13:33 ` David Hildenbrand
2019-03-19 16:04 ` Nitesh Narayan Lal
2019-03-19 17:38 ` Alexander Duyck
2019-03-19 17:59 ` Nitesh Narayan Lal
2019-03-20 13:18 ` Nitesh Narayan Lal
2019-03-25 14:27 ` Nitesh Narayan Lal
2019-03-25 15:37 ` Michael S. Tsirkin
2019-03-25 15:42 ` Nitesh Narayan Lal
2019-03-06 18:00 ` Alexander Duyck
2019-03-06 19:07 ` Nitesh Narayan Lal
2019-03-06 22:05 ` Alexander Duyck
2019-03-07 13:09 ` Nitesh Narayan Lal
2019-03-07 18:45 ` Alexander Duyck
2019-03-07 18:53 ` Michael S. Tsirkin
2019-03-07 19:27 ` David Hildenbrand
2019-03-08 2:24 ` Michael S. Tsirkin
2019-03-08 11:53 ` David Hildenbrand
2019-03-07 21:14 ` Alexander Duyck
2019-03-07 21:28 ` David Hildenbrand
2019-03-07 22:19 ` Alexander Duyck
2019-03-07 19:45 ` Nitesh Narayan Lal
2019-03-07 19:49 ` David Hildenbrand
2019-03-07 18:46 ` Michael S. Tsirkin
2019-03-12 19:58 ` David Hildenbrand
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