3GB Switch setting issue

[edweberg]

Greetings,

I'm running Win XP Pro and I'm trying to properly set the /3GB switch in the boot.ini file.

Original boot.ini:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect

Added this line:
multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Microsoft Windows XP Professional 3GB" /3GB /noexecute=optin /fastdetect

On startup I select the 3GB option. It boots up with a window prompting me to insert the operating system CDs. I don't want to reinstall the OS (I can't).

What do I need to do to get the /3GB switch to work properly on startup? Any Ideas?

Thanks

Erik

 

[Freestone]

Are you using multiple drives and that is the reason rdisk is 1 and not 0?

multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Microsoft Windows XP Professional 3GB" /3GB /noexecute=optin /fastdetect

 

[Zelandakh]

That would explain the booting error - must have been a typo.

 

[edweberg]

I have two lines in the boot.ini

One has rdisk(0) and the other has rdisk(1). I don't know what that means. Maybe it specifies which hard disk the OS is on???

I'll try the /3GB switch with rdisk(0) and see what happens.

Thank you for pointing that out.

Erik

 

[Freestone]

You are partially correct, Erik. rdisk determines which hard drive to boot from. For the technical details, see section MULTI(X) Syntax in the following Knowledge Base article:

http://support.microsoft.com/kb/102873

 

[edweberg]

Wonderful. It booted up just fine with rdisk(0). Now to see if the additional GB of memory allocated to applications makes a noticeable difference. I'm running CAD applications with large file sizes and I'm hoping to see less bogging down. Thanks everyone for the tips.

Erik

 

[strongm]

>I'm hoping to see less bogging down

You may be disappointed:

1) Unless your 32bit app has been especially compiled to make use of that extra 1GB of memory then it won't. Most 32bit apps in Windows can only use 2GB even if the /3GB switch is set. You'll probably need to check with the supplier ...

2) Dropping the kernel mode memory space to 1GB can actually adversely effect performance

 

[edweberg]

That's good stuff to know. Thank you.

Autodesk is actually recommending the /3GB switch, so I'm asumming that their products can use the 3GB. And I'm keeping track of performance. If it is adversely affected, I'll return to the 2GB boot.

If it seems to improve performance I'll recommend the switch to the other power CAD users in our group.

Thanks again,

Erik

 

[Freestone]

And if it seems to improve performance, please post here too.

 

[Bogie9]

Do not expect performance enhancement from setting the /3GB switch. You may expect performance degradation, however, depending on the circumstances.

Setting the /3GB switch increases the virtual address space(VAS) of any given process by reducing the virtual address space reserved by the O/S, WindowsXP in this case. If you do not put in the switch and the the process size(executable/dlls/dataset) grows to nearing 2GB in size, the O/S will kill the process. Basically, the process will abort. With the /3GB switch set, the process can grow to 3GB in VAS. It is a common misunderstanding that more memory means faster. This is not the case with the 3GB switch, the process will only attempt to take the amount of memory it feels it needs depending upon the coding within the executable. If that RAM is not existent, you will have tremendous disk swapping at access times in Milliseconds rather than Nanoseconds. It does not get the 3GB allocated upon initial execution, though. Think of the 3GB VAS as memory addresses that the executable can reference, if indeed it ends up needing that amount of memory. Oh, and by the way, Microsoft is not the only OEM with this O/S issue. A 32-bit application running on HP-UX, Solaris, IRIX, and Linux all experience this VAS limitation, regardless of whether the O/S is 32-bit or 64-bit, and barring PAE which uses the old ECC bits on the memory board to extend to 36 bit. We had similar issues many years ago when transitioning from 8 to 16 bit machines, and from 16 bit to 32 bit machines. Here we are now going to 64 bit from 32 bit in Windows, which has been around a while, but 64 bit has been around for numerous years in the UNIX world. The difference is the amount of VAS the O/S reserves. Solaris is the only O/S that when running a 32-bit application none of the 4GB VAS is reserved by the O/S. This VAS limitation stems from the largest number that can be counted to in a 32-bit binary word, being 4,294,967,295, which results in 4 when divided by 1024 three times.

 

[cdogg]

Bogie...good info. There are some exceptions.

The /3GB switch enables a feature known as 4GT (4-Gigabyte Tuning). Unlike PAE, it actually repartitions the 4GB address space giving apps up to 3GB and stripping the kernel down to 1GB (normally it's evenly divided at 2GB for each). Also a program must contain a flag in its header to take advantage of the extra space. Not many have it. But for the most part it can be a good thing if you have a program that needs it and as long as you're aware of the risk. The kernel is responsible for managing communication between hardware and software. Part of that involves keeping track of memory mappings which you've just increased by using the switch. Now you've got less room for that and other tasks such as memory dumps. Because a lot of programs won't be able to take advantage of the /3GB switch, why bother right? Well Adobe CS2 (and later versions) along with Illustrator are some notable exceptions.


~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Einstein
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