Sp0cky
asked on
What does the /3gb switch do for w2k3 enterprise with say...8 gigs ram? and /3gb questions.
1. Does it still allocate 1 gig for os and the remaining 7 gigs for apps?
2. What does windows do by default (without the /3gb)? Split 4gigs for os (kernel) and 4 for apps?
3. At what point does the amount of memory cause more overhead than benefit for a 32 bit server? 16 gigs?
Thanks.
2. What does windows do by default (without the /3gb)? Split 4gigs for os (kernel) and 4 for apps?
3. At what point does the amount of memory cause more overhead than benefit for a 32 bit server? 16 gigs?
Thanks.
Bertling
this is an interesting read: http://support.microsoft.com/kb/823440
The /3GB switch is only relevant for 32-bit versions of Windows Server 2003. If you have a 64-bit version, the /3gb switch will be ignored.
That being said the 32-bit versions can only work with 4GB of RAM. Under normal scenarios, the kernel gets 2GB of address space while applications each get 2GB of their own address space. The /3GB switch tells the kernel to only take 1 GB of address space, leaving 3GB for applications.
Note, this is address space, and not the amount that actually gets used. For a server with 8 gigs of RAM without the /3GB switch, an application still has only 2GB of address space available. For anything that lies beyond the 2GB boundary, it must remap that extra memory down into its own address space before using it (possibly remapping something else above it that it doesn't need anymore).
The /3GB switch gives it an extra gig to work with at the expense of taking away 1GB for the kernel. Normally, this is not an issue at all, since most server applications do not require the kernel to allocate that much address space. But with a SQL or web server, for example, each connection or pending I/O operation causes the kernel to need extra space allocation for itself, and it would slow things down a lot if it had to start swapping things in and out.
There is a distinction between address space and actual memory that is not easy to understand. Using the /3GB switch does not give a machine with 8 gigs more memory to work with. It just makes management of that memory by the system behave differently.
That being said the 32-bit versions can only work with 4GB of RAM. Under normal scenarios, the kernel gets 2GB of address space while applications each get 2GB of their own address space. The /3GB switch tells the kernel to only take 1 GB of address space, leaving 3GB for applications.
Note, this is address space, and not the amount that actually gets used. For a server with 8 gigs of RAM without the /3GB switch, an application still has only 2GB of address space available. For anything that lies beyond the 2GB boundary, it must remap that extra memory down into its own address space before using it (possibly remapping something else above it that it doesn't need anymore).
The /3GB switch gives it an extra gig to work with at the expense of taking away 1GB for the kernel. Normally, this is not an issue at all, since most server applications do not require the kernel to allocate that much address space. But with a SQL or web server, for example, each connection or pending I/O operation causes the kernel to need extra space allocation for itself, and it would slow things down a lot if it had to start swapping things in and out.
There is a distinction between address space and actual memory that is not easy to understand. Using the /3GB switch does not give a machine with 8 gigs more memory to work with. It just makes management of that memory by the system behave differently.
cuziyg,
just have a question for you sorry to steal the OP but a good qustion:
if i have 2 servers with the /3gb switch
1 has 4gb ram
the other has 8gb ram.
the server with the 8gb ram will have 1 gb for kernal, and 3 for applications.
now does this mean that there is 4gb unused and waisted? will the server with 4gb perform any worse or just the same?
thanks!
just have a question for you sorry to steal the OP but a good qustion:
if i have 2 servers with the /3gb switch
1 has 4gb ram
the other has 8gb ram.
the server with the 8gb ram will have 1 gb for kernal, and 3 for applications.
now does this mean that there is 4gb unused and waisted? will the server with 4gb perform any worse or just the same?
thanks!
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thats a great write up and read thanks a lot, and so well said!
ASKER
Wow, this has to be the best explanation I have heard to a question on this web site. Thank you.