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
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
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.
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
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 ;-)
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
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".
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.
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
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.
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.
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
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 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.
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
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.
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.
On Thu, 2010-07-22 at 17:55 -0400, ken wrote:
On 07/22/2010 05:43 PM JohnS wrote:
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?
--- The 737 is for Programs and Applications use and not for BIOS or Chipsets.
John
On 22/07/10 22:33, ken wrote:
Is the 737M cached (in the output above) what is reserved for bios or chipset...? and what is gained back through remapping?
Nope, it simply means even if I had 2GB RAM, there'd be plenty I'm not using for anything but cache - I don't do much on this laptop but browsing these days :)
What we are suffering from is explained neatly here: http://en.wikipedia.org/wiki/PCI_hole
Simply put, if the BIOS+chipset does not support mapping these reserved areas to higher addresses this memory is lost to us.
My Dell Inspiron 9400 is about 3+ years old now. More recent hardware have the necessary bits and pieces so that you can see all 4GB and use it with 64 bit or 32 bit + PAE. Server chipsets had this kind of support even earlier. Unfortunately for mine, Dell decided to only support 3GB.
From: Hakan Koseoglu hakan@koseoglu.org
In the past, I heard that these 700MB were normally reserved for bios or
chipset 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
Thx for all the answers. As a matter of fact, while my Dell Vostro 410 was getting 3.3GB without PAE, a new Vostro 430 was getting only 3GB (but a full 4GB in PAE)... I went anyway for the non PAE just for peace of mind and because 3GB is still plenty.
Thx, JD
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.
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