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Old 03-22-2011, 04:11 PM
Patrick Bartek
 
Default VirtualBox multi cpu

--- On Tue, 3/22/11, Luc MAIGNAN <luc.maignan@winxpert.com> wrote:

> I use VirtualBox on a RHEL5 with 24 cores (6 x4-cores
> processors).
> But it seems like only one or two processors are used. How
> can I
> configure VirtualBox and/or my virtual machine to allow it
> to use all
> processors ?

Does this not work: Start the VirtualBox interface, choose the virtual machine you want to work on, but don't start it; click on System in the Details list, choose Processor, set Processor(s) slider up to the number of virtual CPUs you want to use.

From the VirtualBox v. 3.1.6 manual:


3.4.2 “Processor” tab

On the “Processor” tab, you can set how many virtual CPU cores the guest operating systems should see. Starting with version 3.0, VirtualBox supports symmetrical multi-processing (SMP) and can present up to 32 virtual CPU cores to each virtual machine.
You should not, however, configure virtual machines to use more CPU cores than you have available physically.



B
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Old 03-22-2011, 04:16 PM
Kevin Martin
 
Default VirtualBox multi cpu

On 03/22/2011 11:39 AM, Genes MailLists wrote:
>> On 03/22/2011 08:57 AM, Luc MAIGNAN wrote:
>> I use VirtualBox on a RHEL5 with 24 cores (6 x4-cores processors).
>> But it seems like only one or two processors are used. How can I
>> configure VirtualBox and/or my virtual machine to allow it to use all
>> processors ?
> Before you start running the VM.
>
> Click on the VM - choose settings->system
>
> Move the slider for how many CPU's you want to allocate to the VM.
>
> gene/
>
>
You may have to use the "taskset" command (google it) to bind the VM's to different CPU's *after* they start up (you could probably
script this in the init.d script fairly easily).

Kevin
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Old 03-22-2011, 04:25 PM
Kevin Martin
 
Default VirtualBox multi cpu

On 03/22/2011 12:11 PM, Patrick Bartek wrote:
> --- On Tue, 3/22/11, Luc MAIGNAN <luc.maignan@winxpert.com> wrote:
>
>> I use VirtualBox on a RHEL5 with 24 cores (6 x4-cores
>> processors).
>> But it seems like only one or two processors are used. How
>> can I
>> configure VirtualBox and/or my virtual machine to allow it
>> to use all
>> processors ?
> Does this not work: Start the VirtualBox interface, choose the virtual machine you want to work on, but don't start it; click on System in the Details list, choose Processor, set Processor(s) slider up to the number of virtual CPUs you want to use.
>
> >From the VirtualBox v. 3.1.6 manual:
>
>
> 3.4.2 “Processor� tab
>
> On the “Processor� tab, you can set how many virtual CPU cores the guest operating systems should see. Starting with version 3.0, VirtualBox supports symmetrical multi-processing (SMP) and can present up to 32 virtual CPU cores to each virtual machine.
> You should not, however, configure virtual machines to use more CPU cores than you have available physically.
>
>
>
> B

It sounds as if the host is not allocating VM's to more than a few CPU's. Adding additional CPU's in the VB screen may or may not
cause the host O.S. to work as planned. It may just allocate more VM virtual CPU's to the same physical CPU's. I'm thinking that
there's a bug in the host O.S. kernel that's not allocating the VM's to all of the CPU's correctly. So "taskset" may be the only
answer until he can get on a newer kernel.

Kevin
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Old 03-22-2011, 05:00 PM
"Christopher A. Williams"
 
Default VirtualBox multi cpu

On Tue, 2011-03-22 at 12:25 -0500, Kevin Martin wrote:
>
> On 03/22/2011 12:11 PM, Patrick Bartek wrote:
> > --- On Tue, 3/22/11, Luc MAIGNAN <luc.maignan@winxpert.com> wrote:
> >
> >> I use VirtualBox on a RHEL5 with 24 cores (6 x4-cores
> >> processors).
> >> But it seems like only one or two processors are used. How
> >> can I
> >> configure VirtualBox and/or my virtual machine to allow it
> >> to use all
> >> processors ?
> > Does this not work: Start the VirtualBox interface, choose the virtual machine you want to work on, but don't start it; click on System in the Details list, choose Processor, set Processor(s) slider up to the number of virtual CPUs you want to use.
> >
> > >From the VirtualBox v. 3.1.6 manual:
> >
> >
> > 3.4.2 “Processor†tab
> >
> > On the “Processor†tab, you can set how many virtual CPU cores the guest operating systems should see. Starting with version 3.0, VirtualBox supports symmetrical multi-processing (SMP) and can present up to 32 virtual CPU cores to each virtual machine.
> > You should not, however, configure virtual machines to use more CPU cores than you have available physically.
> >
> >
> >
> > B
>
> It sounds as if the host is not allocating VM's to more than a few CPU's. Adding additional CPU's in the VB screen may or may not
> cause the host O.S. to work as planned. It may just allocate more VM virtual CPU's to the same physical CPU's. I'm thinking that
> there's a bug in the host O.S. kernel that's not allocating the VM's to all of the CPU's correctly. So "taskset" may be the only
> answer until he can get on a newer kernel.

...Ummm no, that's not how this works.

In reality, allocating more virtual CPUs (aka a vCPU or virtual
processor) to a VM - be it VirtualBox, VMware, Hyper-V, etc., actually
does a have a direct correlation to the number of CPU cores that are
used by that VM on the physical host. The more vCPUs, the more physical
cores are allocated to the VM by the system scheduler foa given set of
CPU cycles (be that the hypervisor, the host OS, or both as
appropriate). That's also why you shouldn't (actually can't) allocate
more vCPUs than the lesser of either the number of CPU cores on the host
system or the total number that the hypervisor will support (32 in the
case of vBox).

Be careful when doing multi-vCPU VMs though. Adding more does not
necessarily mean you will get a boost in performance of either the VM or
the host system. In fact, there are cases where this can actually cause
a performance decrease. Make sure you know that the VM will use the
number of cores (vCPUs) you are wanting to allocate.

This is also something that must be pre-allocated in vBox. Thus, the
correct procedure is to add virtual CPUs via the "Processor" tab as
described above. Other hypervisors (VMware vSphere, Hyper-V) are
starting to support "hot add" of vCPUs on Windows and Linux VMs. None,
however, support a "hot remove" of vCPUs at this time.

Cheers,

Chris
(VMware Certified Advanced Professional - Datacenter Design)

--

======================
"If we don't succeed,
we run the risk of failure."

-- Former President Bill Clinton

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Old 03-22-2011, 06:52 PM
Kevin Martin
 
Default VirtualBox multi cpu

On 03/22/2011 01:00 PM, Christopher A. Williams wrote:
> On Tue, 2011-03-22 at 12:25 -0500, Kevin Martin wrote:
>> On 03/22/2011 12:11 PM, Patrick Bartek wrote:
>>> --- On Tue, 3/22/11, Luc MAIGNAN <luc.maignan@winxpert.com> wrote:
>>>
>>>> I use VirtualBox on a RHEL5 with 24 cores (6 x4-cores
>>>> processors).
>>>> But it seems like only one or two processors are used. How
>>>> can I
>>>> configure VirtualBox and/or my virtual machine to allow it
>>>> to use all
>>>> processors ?
>>> Does this not work: Start the VirtualBox interface, choose the virtual machine you want to work on, but don't start it; click on System in the Details list, choose Processor, set Processor(s) slider up to the number of virtual CPUs you want to use.
>>>
>>> >From the VirtualBox v. 3.1.6 manual:
>>>
>>>
>>> 3.4.2 “Processor†tab
>>>
>>> On the “Processor†tab, you can set how many virtual CPU cores the guest operating systems should see. Starting with version 3.0, VirtualBox supports symmetrical multi-processing (SMP) and can present up to 32 virtual CPU cores to each virtual machine.
>>> You should not, however, configure virtual machines to use more CPU cores than you have available physically.
>>>
>>>
>>>
>>> B
>> It sounds as if the host is not allocating VM's to more than a few CPU's. Adding additional CPU's in the VB screen may or may not
>> cause the host O.S. to work as planned. It may just allocate more VM virtual CPU's to the same physical CPU's. I'm thinking that
>> there's a bug in the host O.S. kernel that's not allocating the VM's to all of the CPU's correctly. So "taskset" may be the only
>> answer until he can get on a newer kernel.
> ...Ummm no, that's not how this works.
>
> In reality, allocating more virtual CPUs (aka a vCPU or virtual
> processor) to a VM - be it VirtualBox, VMware, Hyper-V, etc., actually
> does a have a direct correlation to the number of CPU cores that are
> used by that VM on the physical host. The more vCPUs, the more physical
> cores are allocated to the VM by the system scheduler foa given set of
> CPU cycles (be that the hypervisor, the host OS, or both as
> appropriate). That's also why you shouldn't (actually can't) allocate
> more vCPUs than the lesser of either the number of CPU cores on the host
> system or the total number that the hypervisor will support (32 in the
> case of vBox).
>
> Be careful when doing multi-vCPU VMs though. Adding more does not
> necessarily mean you will get a boost in performance of either the VM or
> the host system. In fact, there are cases where this can actually cause
> a performance decrease. Make sure you know that the VM will use the
> number of cores (vCPUs) you are wanting to allocate.
>
> This is also something that must be pre-allocated in vBox. Thus, the
> correct procedure is to add virtual CPUs via the "Processor" tab as
> described above. Other hypervisors (VMware vSphere, Hyper-V) are
> starting to support "hot add" of vCPUs on Windows and Linux VMs. None,
> however, support a "hot remove" of vCPUs at this time.
>
> Cheers,
>
> Chris
> (VMware Certified Advanced Professional - Datacenter Design)
>
But that's not what the OP said. He said that the machine had 24 cores and the the VM's were being allocated to only a cpu or two.
It sounds like the OP has multiple VM's with multi virtual cpu's per VM setup and that either the virtual machine manager on the
host wasn't allocating the virtual cpu's around correctly or that the kernel is not allocating them correctly. He doesn't mention
which version of VirtualBox (he's not using VMWare) he's using, nor does he mention if he's tried to allocate more virtual cpu's
than would fit on 2 physical cpu's. It's possible that he just doesn't have enough virtual cpu's allocated to need more than 2
physical cpu's in use.

K
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Old 03-22-2011, 07:21 PM
"Christopher A. Williams"
 
Default VirtualBox multi cpu

On Tue, 2011-03-22 at 14:52 -0500, Kevin Martin wrote:
>
> On 03/22/2011 01:00 PM, Christopher A. Williams wrote:
> > On Tue, 2011-03-22 at 12:25 -0500, Kevin Martin wrote:
> >> On 03/22/2011 12:11 PM, Patrick Bartek wrote:
> >>> --- On Tue, 3/22/11, Luc MAIGNAN <luc.maignan@winxpert.com> wrote:
> >>>
> >>>> I use VirtualBox on a RHEL5 with 24 cores (6 x4-cores
> >>>> processors).
> >>>> But it seems like only one or two processors are used. How
> >>>> can I
> >>>> configure VirtualBox and/or my virtual machine to allow it
> >>>> to use all
> >>>> processors ?
> >>> Does this not work: Start the VirtualBox interface, choose the virtual machine you want to work on, but don't start it; click on System in the Details list, choose Processor, set Processor(s) slider up to the number of virtual CPUs you want to use.
> >>>
> >>> >From the VirtualBox v. 3.1.6 manual:
> >>>
> >>>
> >>> 3.4.2 “Processor†tab
> >>>
> >>> On the “Processor†tab, you can set how many virtual CPU cores the guest operating systems should see. Starting with version 3.0, VirtualBox supports symmetrical multi-processing (SMP) and can present up to 32 virtual CPU cores to each virtual machine.
> >>> You should not, however, configure virtual machines to use more CPU cores than you have available physically.
> >>>
> >>>
> >>>
> >>> B
> >> It sounds as if the host is not allocating VM's to more than a few CPU's. Adding additional CPU's in the VB screen may or may not
> >> cause the host O.S. to work as planned. It may just allocate more VM virtual CPU's to the same physical CPU's. I'm thinking that
> >> there's a bug in the host O.S. kernel that's not allocating the VM's to all of the CPU's correctly. So "taskset" may be the only
> >> answer until he can get on a newer kernel.
> > ...Ummm no, that's not how this works.
> >
> > In reality, allocating more virtual CPUs (aka a vCPU or virtual
> > processor) to a VM - be it VirtualBox, VMware, Hyper-V, etc., actually
> > does a have a direct correlation to the number of CPU cores that are
> > used by that VM on the physical host. The more vCPUs, the more physical
> > cores are allocated to the VM by the system scheduler foa given set of
> > CPU cycles (be that the hypervisor, the host OS, or both as
> > appropriate). That's also why you shouldn't (actually can't) allocate
> > more vCPUs than the lesser of either the number of CPU cores on the host
> > system or the total number that the hypervisor will support (32 in the
> > case of vBox).
> >
> > Be careful when doing multi-vCPU VMs though. Adding more does not
> > necessarily mean you will get a boost in performance of either the VM or
> > the host system. In fact, there are cases where this can actually cause
> > a performance decrease. Make sure you know that the VM will use the
> > number of cores (vCPUs) you are wanting to allocate.
> >
> > This is also something that must be pre-allocated in vBox. Thus, the
> > correct procedure is to add virtual CPUs via the "Processor" tab as
> > described above. Other hypervisors (VMware vSphere, Hyper-V) are
> > starting to support "hot add" of vCPUs on Windows and Linux VMs. None,
> > however, support a "hot remove" of vCPUs at this time.
> >
> > Cheers,
> >
> > Chris
> > (VMware Certified Advanced Professional - Datacenter Design)
> >
> But that's not what the OP said. He said that the machine had 24 cores and the the VM's were being allocated to only a cpu or two.
> It sounds like the OP has multiple VM's with multi virtual cpu's per VM setup and that either the virtual machine manager on the
> host wasn't allocating the virtual cpu's around correctly or that the kernel is not allocating them correctly. He doesn't mention
> which version of VirtualBox (he's not using VMWare) he's using, nor does he mention if he's tried to allocate more virtual cpu's
> than would fit on 2 physical cpu's. It's possible that he just doesn't have enough virtual cpu's allocated to need more than 2
> physical cpu's in use.

You missed it in my explanation, me thinks...

The OP said that his HOST system had a total of 24 cores (assume he has
a 4-socket, 6-core system). He wanted to know why his VMs were not being
allocated to more than 1 or 2 physical CPUs on his host system.

That he is running vBox, as opposed to VMware (or something else), makes
absolutely NO DIFFERENCE here. VMware, vBox, Hyper-V, and others all
handle this part very much the same way, and have for a very long time.
The version of vBox won't matter either. I've already explained why this
(what amounts to virtual SMP) works the way it does, and I won't repeat
that explanation.

He would have to create a VM with at least 13 vCPUs in order to
guarantee that things would get scheduled on more than 2 CPUs at a time
on his host server. Even with several multi-vCPU VMs, if none of them
had more than 6 vCPUs, it is both possible and plausible that you would
not allocate resources to more than even 1 physical CPU on the host if
CPU utilization is low on all of the VMs.

That's just the way hypervisor schedulers work. Some are arguably better
at it than others and also have individual features others won't, but
the basics of virtual SMP are going to be pretty much the same across
the board.

Chris


--

======================
"Never murder a man when he's
busy committing suicide."

-- Woodrow Wilson

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Old 03-24-2011, 06:47 AM
Luc MAIGNAN
 
Default VirtualBox multi cpu

Yes, indeed....

I've modified the number of cpus via the command line :

VBoxManage modifyvm MyVM --cpus x

From 3 cpus, the performances go down... But I don't really understand
why... If someone can give me ways of explanations...

Le 22/03/11 20:52, Kevin Martin a écrit :
>
> On 03/22/2011 01:00 PM, Christopher A. Williams wrote:
>> On Tue, 2011-03-22 at 12:25 -0500, Kevin Martin wrote:
>>> On 03/22/2011 12:11 PM, Patrick Bartek wrote:
>>>> --- On Tue, 3/22/11, Luc MAIGNAN<luc.maignan@winxpert.com> wrote:
>>>>
>>>>> I use VirtualBox on a RHEL5 with 24 cores (6 x4-cores
>>>>> processors).
>>>>> But it seems like only one or two processors are used. How
>>>>> can I
>>>>> configure VirtualBox and/or my virtual machine to allow it
>>>>> to use all
>>>>> processors ?
>>>> Does this not work: Start the VirtualBox interface, choose the virtual machine you want to work on, but don't start it; click on System in the Details list, choose Processor, set Processor(s) slider up to the number of virtual CPUs you want to use.
>>>>
>>>> > From the VirtualBox v. 3.1.6 manual:
>>>>
>>>>
>>>> 3.4.2 “Processor†tab
>>>>
>>>> On the “Processor†tab, you can set how many virtual CPU cores the guest operating systems should see. Starting with version 3.0, VirtualBox supports symmetrical multi-processing (SMP) and can present up to 32 virtual CPU cores to each virtual machine.
>>>> You should not, however, configure virtual machines to use more CPU cores than you have available physically.
>>>>
>>>>
>>>>
>>>> B
>>> It sounds as if the host is not allocating VM's to more than a few CPU's. Adding additional CPU's in the VB screen may or may not
>>> cause the host O.S. to work as planned. It may just allocate more VM virtual CPU's to the same physical CPU's. I'm thinking that
>>> there's a bug in the host O.S. kernel that's not allocating the VM's to all of the CPU's correctly. So "taskset" may be the only
>>> answer until he can get on a newer kernel.
>> ...Ummm no, that's not how this works.
>>
>> In reality, allocating more virtual CPUs (aka a vCPU or virtual
>> processor) to a VM - be it VirtualBox, VMware, Hyper-V, etc., actually
>> does a have a direct correlation to the number of CPU cores that are
>> used by that VM on the physical host. The more vCPUs, the more physical
>> cores are allocated to the VM by the system scheduler foa given set of
>> CPU cycles (be that the hypervisor, the host OS, or both as
>> appropriate). That's also why you shouldn't (actually can't) allocate
>> more vCPUs than the lesser of either the number of CPU cores on the host
>> system or the total number that the hypervisor will support (32 in the
>> case of vBox).
>>
>> Be careful when doing multi-vCPU VMs though. Adding more does not
>> necessarily mean you will get a boost in performance of either the VM or
>> the host system. In fact, there are cases where this can actually cause
>> a performance decrease. Make sure you know that the VM will use the
>> number of cores (vCPUs) you are wanting to allocate.
>>
>> This is also something that must be pre-allocated in vBox. Thus, the
>> correct procedure is to add virtual CPUs via the "Processor" tab as
>> described above. Other hypervisors (VMware vSphere, Hyper-V) are
>> starting to support "hot add" of vCPUs on Windows and Linux VMs. None,
>> however, support a "hot remove" of vCPUs at this time.
>>
>> Cheers,
>>
>> Chris
>> (VMware Certified Advanced Professional - Datacenter Design)
>>
> But that's not what the OP said. He said that the machine had 24 cores and the the VM's were being allocated to only a cpu or two.
> It sounds like the OP has multiple VM's with multi virtual cpu's per VM setup and that either the virtual machine manager on the
> host wasn't allocating the virtual cpu's around correctly or that the kernel is not allocating them correctly. He doesn't mention
> which version of VirtualBox (he's not using VMWare) he's using, nor does he mention if he's tried to allocate more virtual cpu's
> than would fit on 2 physical cpu's. It's possible that he just doesn't have enough virtual cpu's allocated to need more than 2
> physical cpu's in use.
>
> K

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Old 03-24-2011, 10:46 AM
Ian Chapman
 
Default VirtualBox multi cpu

On 23/03/11 02:00, Christopher A. Williams wrote:

> This is also something that must be pre-allocated in vBox. Thus, the
> correct procedure is to add virtual CPUs via the "Processor" tab as
> described above. Other hypervisors (VMware vSphere, Hyper-V) are
> starting to support "hot add" of vCPUs on Windows and Linux VMs. None,
> however, support a "hot remove" of vCPUs at this time.

I believe VirtualBox does support hot add and remove of processors.

--
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Old 03-24-2011, 11:18 AM
"Christopher A. Williams"
 
Default VirtualBox multi cpu

On Thu, 2011-03-24 at 08:47 +0100, Luc MAIGNAN wrote:
> Yes, indeed....
>
> I've modified the number of cpus via the command line :
>
> VBoxManage modifyvm MyVM --cpus x
>
> From 3 cpus, the performances go down... But I don't really understand
> why... If someone can give me ways of explanations...

Sure...! This has to do with the way hypervisor schedulers work with
virtual SMB. In your case, the VM you are running may simply not need or
use more than 2 CPU cores, or assigning 3 or more cores causes the
scheduler to have to prioritize things in a less efficient way. Perhaps
both are happening as well. Here's how this works:

Basically, a multi-vCPU VM has to be scheduled for the same number of
CPU cores for every CPU cycle (2 vCPUs, 2 cores; 3 vCPUs, 3 cores; and
so forth). And this part is absolutely key: This gets done regardless of
if there is actually anything for these cores to do.

So, if you have a 4 vCPU VM that only has work to do for a single core,
you still have to schedule 4 cores - of which 3 cores will be scheduled
to basically do ...NOTHING! These cores cannot be used to perform other
work during this time.

I hope you can see that scheduling CPU cores to do nothing could cause a
performance decrease. You basically are forcing the scheduler to choose
which will do nothing as well. Further, scheduling vCPUs in odd numbers
(like 3 vCPUs on a quad-core processor) causes other effects on
performance due to scheduling conflicts. Say, for example, I have a 3
vCPU VM and a 2 vCPU VM running on a quad-core processor. I can never
schedule these two to execute at the same time because I don't have
enough cores.

So, the general rule of thumb for how many vCPUs you need on a VM goes
something like this:

Highly threaded systems and applications will tend to need - and use -
more cores, so configure more vCPUs for them. Things that tend to be
heavy on calculations will tend to need fewer cores. Also, configure
your VMs taking the total number of cores you have on the system into
account so as to minimize possible scheduling conflicts. I usually use
multiples of 2 vCPUs (1, 2, 4, 8) and avoid odd numbered vCPU
configurations. That's especially true on quad-core processors. Six-core
processors give you a little more flexibility (you could do combinations
of 1, 2, 4, 6), but the same general rule still applies.

In any case, make sure you understand the actual CPU needs of the system
and application you are running. I have seen a single vCPU VM use more
processor time on a dual-socket, quad-core box than an 8-way VM running
on the same machine at the same time. In that case, the 8-way VM was
happy as a clam because it wasn't that busy, but the single vCPU VM was
redlining the processor because it wanted even more than what physical
resources were available.

Hope that helps explain things for you!

Chris


--

==============================
"If you are calm while all around you is chaos,
then you probably haven't fully understood
the magnitude of the situation."

--Unknown

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Old 03-24-2011, 11:29 AM
"Christopher A. Williams"
 
Default VirtualBox multi cpu

On Thu, 2011-03-24 at 19:46 +0800, Ian Chapman wrote:
> On 23/03/11 02:00, Christopher A. Williams wrote:
>
> > This is also something that must be pre-allocated in vBox. Thus, the
> > correct procedure is to add virtual CPUs via the "Processor" tab as
> > described above. Other hypervisors (VMware vSphere, Hyper-V) are
> > starting to support "hot add" of vCPUs on Windows and Linux VMs. None,
> > however, support a "hot remove" of vCPUs at this time.
>
> I believe VirtualBox does support hot add and remove of processors.

...Indeed!

Must have missed that update. It's right here in the documentation:
http://www.virtualbox.org/manual/ch09.html#cpuhotplug

You do need to pre-configure the VM to enable it, as well as specify the
max number of vCPUs it can have. But once you've done that, you're good
to go.

Thanks for pointing it out.

Chris

--

======================
"If we don't succeed,
we run the risk of failure."

-- Former President Bill Clinton

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