Memory Management - Demystifying /3GB

Comments

# bitzie said on March 26, 2007 3:52 PM:

...but not on a SBS box.

Please do not use the /3 switch there.

# Clive Watson's Weblog said on March 27, 2007 7:53 AM:

...I Hope! Well done to the performance team for their recent Windows Server articles, in particular

# Active Directory, Cluster and other fun stuff... said on March 28, 2007 8:13 AM:

I see a lot of customers unnecessarily using the boot.ini /3GB switch. Explaining when and when not to

# Jeff Louque said on March 30, 2007 11:19 AM:

Keep in mind that the /Userva switch was introduced in Windows 2003 and does not work with Windows 2000.

# Brad Rutkowski said on March 30, 2007 5:05 PM:

You should aslo mention that 3GB and PAE dont like to play nice and should not be enabled together.

# Jason Boche said on April 3, 2007 2:38 PM:

"You should aslo mention that 3GB and PAE dont like to play nice and should not be enabled together."

That has not been my experience, but we only used the /PAE and /3GB tuning switch on one particular set of SQL cluster servers.

# bday said on April 3, 2007 3:32 PM:

"Brad Rutkowski said:  

You should aslo mention that 3GB and PAE dont like to play nice and should not be enabled together."

This is incorrect. Our Exchange 2003 servers use the following and are quite happy.

/PAE /3GB /USERVA=3030* /BASEVIDEO

*2970 in some cases after some PTE tuning

Without /PAE we never see over 3.75GB of physical memory installed on this hardware set, so it helps out a lot.

# Confused said on April 3, 2007 4:17 PM:

>/3GB won't enable you to see the additional 4GB or 8GB of RAM you added to your new server

>/3GB doesn't necessarily make your application 50% more efficient

>/3GB should not be a standard for your environment (there are exceptions, and we'll get to those)

From the writing, I can't immediately tell: are these the myths? Or are these the "dispellations" ?

# CC Hameed said on April 3, 2007 5:06 PM:

The two sentences preceding the ones you cited should provide the context:

"At least once a week we have this discussion with a Systems Administrator somewhere who has this set in the boot.ini file on all of the servers but doesn't know why.  Maybe someone added it to the server build process at some point or perhaps someone read about the /3GB switch somewhere and thought that it would improve performance or enable them to see the full 4GB or 8GB of physical memory installed on the system"

So, if you think of the bullet points as factual statements:

- /3GB should not be a standard for your environment (except in certain circumstances)

- /3GB won't enable you to see the additional 4GB or 8GB of RAM

- /3GB does not necessarily make your application 50% more efficient

Hope this clears up the misunderstanding!

Thanks for the feedback

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# Billy Dhillon said on April 11, 2007 10:37 AM:

""Brad Rutkowski said:  

You should aslo mention that 3GB and PAE dont like to play nice and should not be enabled together."

This is incorrect. Our Exchange 2003 servers use the following and are quite happy.

/PAE /3GB /USERVA=3030* /BASEVIDEO

*2970 in some cases after some PTE tuning

Without /PAE we never see over 3.75GB of physical memory installed on this hardware set, so it helps out a lot."

So I think Brad is going back to a issue back from Windows 2000 Server. Their use to be a issue when you addeded /3GB & /PAE together on DataCenter edition with more then 16 GB's of ram, it would cap you down to 16GB, regardless if you had More then that for memory..

Another issue with having both enabled is that /PAE increases the size of your PTE Size from 4 bytes to 8 Bytes...So when you have /3GB & /PAE together, in certain situations you can hurt yourself...

For Example..../3GB cuts down your nonpaged and pool paged memory and PAE will double your PTE Size. So if you do a Large File copy using the "Copy" command...You can incur a performance problem because you will be consuming Pool Memory at a faster rate because your PTE size is increased while your pool memory has decreased in size which might result in failed copies and depelted pool memory.

But on another note, in Windows 2003 SP1 (and servers with Hot-Add Memory), Windows will load the ntrknlpa (thats PAE kernel) by default. So essentialy you have PAE running on any newly built 2K3 Server with SP1+

# Nudge said on April 11, 2007 6:07 PM:

In a Windows 2003 server with sp1 and 16GB of RAM, I still need to use /PAE to allow the OS to address more than 3.75GB of RAM.

In this case is there any benefit to using /3GB?  As there is more than enough RAM anyway, is there any point to limiting the Kernel to 1GB of RAM?

I would anticipate performance problems in this case.

What are everyone's thoughts?

# Billy Dhillon said on April 12, 2007 10:49 AM:

"In a Windows 2003 server with sp1 and 16GB of RAM, I still need to use /PAE to allow the OS to address more than 3.75GB of RAM.

In this case is there any benefit to using /3GB?  As there is more than enough RAM anyway, is there any point to limiting the Kernel to 1GB of RAM?

I would anticipate performance problems in this case.

What are everyone's thoughts?"

I've seen that Issue on DEP enabled Proc's where you need to add the /PAE switch even if SP1 is installed (is this a HP Server by chance?).

As for adding the /3GB, what is the role of this server? You have to remember that /3GB is virtual Memory which is different the Physical memory. The Memory managment model works only in a 2GB/2GB (or 3GB/1GB) form. If their is nothing on your server that would take advantage of /3GB (any app that utilizes /largeaddressaware), then it would not benfit at all, it would likely degrade the performance of your box.

# xentelworker said on April 12, 2007 11:43 AM:

"In a Windows 2003 server with sp1 and 16GB of RAM, I still need to use /PAE to allow the OS to address more than 3.75GB of RAM.

In this case is there any benefit to using /3GB?  As there is more than enough RAM anyway, is there any point to limiting the Kernel to 1GB of RAM?

I would anticipate performance problems in this case.

What are everyone's thoughts?"

If you don't have a DEP enabled processor, you will still need to add /PAE even if your have upgraded to SP1+

(Ref: http://support.microsoft.com/default.aspx?scid=kb;en-us;875352)

As for adding the /3GB, what is the role of this server?

You have to remember that /3GB is virtual Memory which is different the Physical memory. The Memory managment model works only in a 2GB/2GB (or 3GB/1GB) form.

If their is nothing on your server that would take advantage of /3GB (any app that utilizes /largeaddressaware), then it would not benfit at all, it would likely degrade the performance of your box.

# Ask the Performance Team said on May 15, 2007 6:02 AM:

This is a follow-up to our Processes consuming high amounts of virtual memory post from January. Pagefile

# Sashank said on May 21, 2007 4:59 PM:

Is there some way to (programatically) find out when the OS is booted in the /3GB mode? The 32-bit version of my application sets a hard limit of 1700 MB after which it does not try to allocate any more memory. I'd like to switch this to 2700 MB when the /3GB switch is available. The best I can come up with till now is to read the boot.ini file, but I am wondering if there is anything clever out there, that I can do.

Thank you for the elaborate discussions on Windows Memory.

Sashank.

# Digvijay said on June 21, 2007 11:04 AM:

Nice Post..

helped me to clear a lot of doubt about the /3GB switch

Keep up the good work, Tim

# xelox said on August 29, 2007 8:21 PM:

Sashank: You can use powershell/wmi to get the value from boot.ini.

# Gonzalo said on September 22, 2007 9:31 PM:

Hi.

I Have a Windows 2003 SP2 Terminal Server with 2 GB in RAM and i haven't found a good advice in order to use (or not) the /3GB /Userva:3030 in a Terminal Server.

# Ask the Performance Team said on September 25, 2007 10:04 AM:

Following on from our post on troubleshooting a basic application crash , it's time to start troubleshooting

# Ask the Performance Team said on September 28, 2007 10:35 AM:

In previous posts, we've discussed the Basics of Memory Management , Pool Resources and of course the

# Tim Newton said on October 5, 2007 12:37 PM:

In response to Gonzalo's question, the answer is NO. You do not want to enable /3GB or USERVA unless you have a specific application need to do so, such as in an Exchange environment. On a terminal server, the last thing you want to do is cripple the kernel by enabling /3GB.

Gonzalo posted:

"I Have a Windows 2003 SP2 Terminal Server with 2 GB in RAM and i haven't found a good advice in order to use (or not) the /3GB /Userva:3030 in a Terminal Server."

# Richard said on October 9, 2007 10:10 AM:

Hi,

My servers have 17 GB of RAM and the /3GB /PAE switches enabled (vendors recommendation).

The BIOS shows 17 GB, but the OS only displays 15 GB.  I thought the /3GB would limit to 16, but why 15 GB? Any ideas?

# John said on October 23, 2007 5:43 PM:

You wrote about Microsoft Exchange servers. Using Catia, a high end auto / aero CAD package, the recomendation has been to run 3 GB by default. Is this a problem? We commonly work with data files in the hundreds of MBs

# Ask the Performance Team said on October 30, 2007 9:14 AM:

In previous posts we've discussed the basics of memory management including an overview of kernel and

# Louis said on January 15, 2008 6:09 PM:

Is the /3GB Switch required in the 32-Bit Version of Windows 2003 R2????

# wizualizacje architektoniczne said on February 4, 2008 9:27 AM:

Thanks for that info.

Regards

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# MarkR said on February 22, 2008 2:15 PM:

After running the EXBPA, it recomends adding the /3GB to the Win2003 domain contollers in addition to the Exchange 2003 mailbox servers.  What are the benifits to adding the /3GB to the DCs?

# Cmonurz said on May 10, 2008 6:37 AM:

Hi guy's, we get asked this a lot in the Terminal Services/Citrix world, where misguides admins add /3GB switch and cripple their servers.

Can you guy's confirm that in a 32bit Windows 2003 terminal services environment that this is a totally inappropriate modification?

Thanks!

# Krishna said on May 14, 2008 11:57 PM:

If my server has 32GB of RAM, is it possible to make IIS use all 32GB of RAM ?

# Werner said on May 30, 2008 3:08 PM:

So what is the maxinum amount of RAM that will work on a Intel Core2Duo with Win XP SP2 without any going to waste?

# Manvendra Kumar said on June 30, 2008 4:19 AM:

I have 4GB of ram installed and 3gb switch enabled. I am running SQL 2000. Few days back my server crashed. Is there any possibilities that it could have been caused due to 3gb switch limit settings. I couldn't find any errors through logs, event and dump analysis. If yes, what kind of error will indicate that it was caused due 3gb switch settings.

# Anil said on July 13, 2008 7:07 AM:

When configuring a new Exchange 2003 SP2 server on a Windows 2003 server with 4GB of RAM, what are the three settings that need to be changed in Windows to optimize memory?

# Paulo Oliveira said on August 28, 2008 10:05 AM:

So, this means that I don´t need the /3GB switch to make windows reconize up to 4GB? The OS will reconize automaticly?

Regards,

Paulo Oliveira.

# timothyn said on September 2, 2008 5:34 PM:

Paulo,

 Yes, that is correct. The /3GB switch has absolutely nothing to do with how much physical RAM you have installed in the machine, simply how much VIRTUAL memory is allocated to user mode. The confusion over this is why I wrote this post in the first place.

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