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Old 07-22-2010, 09:25 AM
John Doe
 
Default To PAE or not to PAE...

Hi,

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...
In the past, I heard that these 700MB were normally reserved for bios or chipset
stuff...
And that running in PAE would slow down some processes.
By the way, I know 64 bits would solve this dilemn but right now I am 32bits...

Thx,
JD



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Old 07-22-2010, 11:48 AM
Peter Kjellstrom
 
Default To PAE or not to PAE...

On Thursday 22 July 2010, John Doe wrote:
> Hi,
>
> 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...
> In the past, I heard that these 700MB were normally reserved for bios or
> chipset stuff...
> And that running in PAE would slow down some processes.
> By the way, I know 64 bits would solve this dilemn but right now I am
> 32bits...

There is no general answer. Getting more memory could for some workloads be
critical. On the other hand, other loads might not care much about the extra
few hundred megs but instead suffer some PAE related slowdown.

Good news though, trivial to test. Just reboot with the PAE kernel, check the
performance of your workload. If you see improvment stay on PAE, else go
back.

/Peter
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Old 07-22-2010, 05:07 PM
Warren Young
 
Default To PAE or not to PAE...

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?

Is your CPU 64-bit capable? That's generally a better idea than PAE.
Keep in mind that PAE is Pentium Pro era technology.
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Old 07-22-2010, 05:31 PM
Kevin Krieser
 
Default To PAE or not to PAE...

But it would allow other usages of that RAM. cache. Other programs with great memory usage.

Of course, as mentioned earlier, you would have to test with your workload whether the extra overhead is more than made up with the extra memory availability.

On Jul 22, 2010, at 12:07 PM, 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?
>
> Is your CPU 64-bit capable? That's generally a better idea than PAE.
> Keep in mind that PAE is Pentium Pro era technology.
> _______________________________________________
> CentOS mailing list
> CentOS@centos.org
> http://lists.centos.org/mailman/listinfo/centos

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Old 07-22-2010, 05:51 PM
Markus Falb
 
Default 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

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Old 07-22-2010, 05:58 PM
"Joseph L. Casale"
 
Default To PAE or not to PAE...

>What is a program supposed to do to get at "that extra RAM" then ?
>Just curious ;-)

AFAIK, it must be specifically compiled for it...
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Old 07-22-2010, 06:28 PM
JohnS
 
Default To PAE or not to PAE...

On Thu, 2010-07-22 at 17:58 +0000, 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...
---

A PAE enabled Kernel:

NO. It is dependent on the mmap() call in the program as in how the
memory allocation is done in it.
dmesg | grep Memory
Memory: 2070724k/2097024k available (2254k kernel code, 25172k reserved,
1149k data, 272k init, 1179520k highmem)

cat /proc/mtrr
reg00: base=0x00000000 ( 0MB), size=2048MB: write-back, count=1
reg01: base=0xfc000000 (4032MB), size= 64MB: uncachable, count=1
reg02: base=0xfd000000 (4048MB), size= 8MB: write-combining, count=1

Total of Memory Array(s)
Attributes : Total Installed Capacity
Value : 2048 MB

Attributes : Total Installed Capacity Available to the OS
Value : 2024 MB

For as performance wise some would be amazed at what a 32bit machine on
pae can do (prog dependent). Memory allocation is faster than a X86_64.
IE locking spin lock... Don't be confused there is a point to stop and
go 64Bit only.

John



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Old 07-22-2010, 06:39 PM
Hakan Koseoglu
 
Default To PAE or not to PAE...

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

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Old 07-22-2010, 06:43 PM
Hakan Koseoglu
 
Default To PAE or not to PAE...

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.
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Old 07-22-2010, 06:50 PM
Markus Falb
 
Default To PAE or not to PAE...

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".

--
Regards, Markus

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