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Old 07-22-2010, 06:52 PM
Keith Roberts
 
Default To PAE or not to PAE...

On Thu, 22 Jul 2010, Markus Falb wrote:

> To: centos@centos.org
> From: Markus Falb <markus.falb@fasel.at>
> Subject: Re: [CentOS] To PAE or not to PAE...
>
> On 22/07/2010 19:07, Warren Young wrote:
>> On 7/22/2010 3:25 AM, John Doe wrote:
>>>
>>> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
>>> those exta 700MB...
>>
>> Very few programs can use PAE to get at that extra RAM. Can the
>> programs you run do this?
>
> What is a program supposed to do to get at "that extra RAM" then ?
> Just curious ;-)
>
> --
> Regards, Markus

Here's an explanation on what PAE is and how it works:

http://en.wikipedia.org/wiki/Physical_Address_Extension

Kind Regards,

Keith
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Old 07-22-2010, 06:56 PM
JohnS
 
Default To PAE or not to PAE...

On Thu, 2010-07-22 at 19:43 +0100, Hakan Koseoglu wrote:
> On 22/07/10 18:07, Warren Young wrote:
> > Very few programs can use PAE to get at that extra RAM. Can the
> > programs you run do this?
> With PAE you can utilize all of the extra RAM but each individual
> program will be limited to 3GB user space, you can fit a more of them
> into 16GB, still using 32 bit. You won't need additional coding to do
> that. Using more than 3-4GB per process will require such fiddlings.
---
Try about 69Gbytes & What are you fiddling with? limits.conf?

John

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Old 07-22-2010, 07:15 PM
Drew
 
Default To PAE or not to PAE...

>> I was wondering if anyone would know the cons of running a PAE kernel...?
>> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
>> those exta 700MB...
> You should use 64 bit if possible but if you're seeing 3.2GB, it's more
> likely that your motherboard is not capable (I have one of those here
> right now).
>
>> In the past, I heard that these 700MB were normally reserved for bios or chipset
>> stuff...
> It still is, even with with 64 bit. If your motherboard supports
> remapping this memory with 64 bit you can use the whole 4GB. Otherwise
> you're limited to 3.2:

Second this. It doesn't matter if you use PAE or x86-64. Without
support in the bios for remapping the lost memory you'll never recover
that extra RAM.


--
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie
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Old 07-22-2010, 07:25 PM
Les Mikesell
 
Default To PAE or not to PAE...

On 7/22/2010 1:50 PM, Markus Falb wrote:
> On 22/07/2010 19:58, Joseph L. Casale wrote:
>>> What is a program supposed to do to get at "that extra RAM" then ?
>>> Just curious ;-)
>>
>> AFAIK, it must be specifically compiled for it...
>
> I always thought PAE is quite transparent thing for user programs. PAE
> is a hardware feature and hardware management is a kernel task after
> all, isnt it ? The kernel has to be compiled for PAE, of course.
>
> What I want to say: I do not know about "programming techniqes to
> exploit PAE".

Make the decision a lot easier: add another 4 gig of RAM to the box and
install the PAE kernel. win/win. For just 4 gigs it's probably a wash
anyway.

--
Les Mikesell
lesmikesell@gmail.com
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Old 07-22-2010, 09:05 PM
Markus Falb
 
Default To PAE or not to PAE...

On 22/07/2010 11:25, John Doe wrote:
> Hi,
>
> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
> those exta 700MB...
> In the past, I heard that these 700MB were normally reserved for bios or chipset
> stuff...

I installed a 32 bit centos and with the non-pae kernel it shows 3.3 gb
for me too. I had a look at bios but there is nothing of interest,
nothing obvious at least. Asking myself if it is "normally" as you said.

--
Regards, Markus

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Old 07-22-2010, 09:33 PM
ken
 
Default To PAE or not to PAE...

On 07/22/2010 02:39 PM Hakan Koseoglu wrote:
> JD,
> On 22/07/10 10:25, John Doe wrote:
>> I was wondering if anyone would know the cons of running a PAE kernel...?
>> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
>> those exta 700MB...
> You should use 64 bit if possible but if you're seeing 3.2GB, it's more
> likely that your motherboard is not capable (I have one of those here
> right now).
>
>> In the past, I heard that these 700MB were normally reserved for bios or chipset
>> stuff...
> It still is, even with with 64 bit. If your motherboard supports
> remapping this memory with 64 bit you can use the whole 4GB. Otherwise
> you're limited to 3.2:
>
> hakan@photon:~$ free -m
> total used free shared buffers cached
> Mem: 3262 1972 1290 0 103 737
> -/+ buffers/cache: 1131 2131
> Swap: 7812 308 7504
> hakan@photon:~$ arch
> x86_64

I'm trying to catch up...

Is the 737M cached (in the output above) what is reserved for bios or
chipset...? and what is gained back through remapping?

tnx
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Old 07-22-2010, 09:43 PM
JohnS
 
Default To PAE or not to PAE...

On Thu, 2010-07-22 at 17:33 -0400, ken wrote:
> On 07/22/2010 02:39 PM Hakan Koseoglu wrote:
> > JD,
> > On 22/07/10 10:25, John Doe wrote:
> >> I was wondering if anyone would know the cons of running a PAE kernel...?
> >> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
> >> those exta 700MB...
> > You should use 64 bit if possible but if you're seeing 3.2GB, it's more
> > likely that your motherboard is not capable (I have one of those here
> > right now).
> >
> >> In the past, I heard that these 700MB were normally reserved for bios or chipset
> >> stuff...
> > It still is, even with with 64 bit. If your motherboard supports
> > remapping this memory with 64 bit you can use the whole 4GB. Otherwise
> > you're limited to 3.2:
> >
> > hakan@photon:~$ free -m
> > total used free shared buffers cached
> > Mem: 3262 1972 1290 0 103 737
> > -/+ buffers/cache: 1131 2131
> > Swap: 7812 308 7504
> > hakan@photon:~$ arch
> > x86_64
>
> I'm trying to catch up...
>
> Is the 737M cached (in the output above) what is reserved for bios or
> chipset...? and what is gained back through remapping?
---
If you would look at my prior post you can actually see it as in whats
used. It is 2GB of ram on 12GB capability.

Run "dmesg | grep Memory" on your machine to see it.

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Old 07-22-2010, 09:43 PM
JohnS
 
Default To PAE or not to PAE...

On Thu, 2010-07-22 at 17:33 -0400, ken wrote:
> On 07/22/2010 02:39 PM Hakan Koseoglu wrote:
> > JD,
> > On 22/07/10 10:25, John Doe wrote:
> >> I was wondering if anyone would know the cons of running a PAE kernel...?
> >> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
> >> those exta 700MB...
> > You should use 64 bit if possible but if you're seeing 3.2GB, it's more
> > likely that your motherboard is not capable (I have one of those here
> > right now).
> >
> >> In the past, I heard that these 700MB were normally reserved for bios or chipset
> >> stuff...
> > It still is, even with with 64 bit. If your motherboard supports
> > remapping this memory with 64 bit you can use the whole 4GB. Otherwise
> > you're limited to 3.2:
> >
> > hakan@photon:~$ free -m
> > total used free shared buffers cached
> > Mem: 3262 1972 1290 0 103 737
> > -/+ buffers/cache: 1131 2131
> > Swap: 7812 308 7504
> > hakan@photon:~$ arch
> > x86_64
>
> I'm trying to catch up...
>
> Is the 737M cached (in the output above) what is reserved for bios or
> chipset...? and what is gained back through remapping?
---
If you would look at my prior post you can actually see it as in whats
used. It is 2GB of ram on 12GB capability.

Run "dmesg | grep Memory" on your machine to see it.

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Old 07-22-2010, 09:55 PM
ken
 
Default To PAE or not to PAE...

On 07/22/2010 05:43 PM JohnS wrote:
> On Thu, 2010-07-22 at 17:33 -0400, ken wrote:
>> On 07/22/2010 02:39 PM Hakan Koseoglu wrote:
>>> JD,
>>> On 22/07/10 10:25, John Doe wrote:
>>>> I was wondering if anyone would know the cons of running a PAE kernel...?
>>>> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
>>>> those exta 700MB...
>>> You should use 64 bit if possible but if you're seeing 3.2GB, it's more
>>> likely that your motherboard is not capable (I have one of those here
>>> right now).
>>>
>>>> In the past, I heard that these 700MB were normally reserved for bios or chipset
>>>> stuff...
>>> It still is, even with with 64 bit. If your motherboard supports
>>> remapping this memory with 64 bit you can use the whole 4GB. Otherwise
>>> you're limited to 3.2:
>>>
>>> hakan@photon:~$ free -m
>>> total used free shared buffers cached
>>> Mem: 3262 1972 1290 0 103 737
>>> -/+ buffers/cache: 1131 2131
>>> Swap: 7812 308 7504
>>> hakan@photon:~$ arch
>>> x86_64
>> I'm trying to catch up...
>>
>> Is the 737M cached (in the output above) what is reserved for bios or
>> chipset...? and what is gained back through remapping?
> ---
> If you would look at my prior post you can actually see it as in whats
> used. It is 2GB of ram on 12GB capability.
>
> Run "dmesg | grep Memory" on your machine to see it.

What do you mean above in the three instances of "it"? Don't know what
you're referring to.


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Old 07-22-2010, 10:13 PM
Kevin Krieser
 
Default To PAE or not to PAE...

On Jul 22, 2010, at 12:51 PM, Markus Falb wrote:

> On 22/07/2010 19:07, Warren Young wrote:
>> On 7/22/2010 3:25 AM, John Doe wrote:
>>>
>>> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
>>> those exta 700MB...
>>
>> Very few programs can use PAE to get at that extra RAM. Can the
>> programs you run do this?
>
> What is a program supposed to do to get at "that extra RAM" then ?
> Just curious ;-)

If it is supported by your chipset, etc, then the extra RAM just goes into the pool of available RAM for system usage. An extra 700MB of program space that doesn't have to be paged out, or discard clean pages, depending on overall memory pressure of the system.

But system calls are costlier, since the kernel has to do more work swapping into different address spaces. For instance, on a 3GB RAM computer with a 32 bit OS, the OS can just access any physical RAM directly. But since the OS is 32 bit too, it has more work to access "upper" RAM.
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