32 Vs 64 guest

[PaulVM]

Are there advantage/disadvantages on using 32 o 64 bit guests (CentOS 6.x in my mind at the moment)?

Atually my host had 16 GB ram. I don't think I need more than 4 GB ram for a couple of years in my guests, but I am interested in knowing if there are performance difference between 32 and 64 guest VMs.

Regards, P.

 

[JohnD]

Are there advantage/disadvantages on using 32 o 64 bit guests (CentOS 6.x in my mind at the moment)?

Atually my host had 16 GB ram. I don't think I need more than 4 GB ram for a couple of years in my guests, but I am interested in knowing if there are performance difference between 32 and 64 guest VMs.

Regards, P.

Afaik:

Use 32bit if and ONLY if you have not enough RAM. Because 64bit needs more RAM and not enough RAM = massive performance issues.
You will have same or better performance with 64 bit as long as you have enough RAM (performance benefits depends on the application, will be something between 0% and 30%).

 

[mir]

Are there advantage/disadvantages on using 32 o 64 bit guests (CentOS 6.x in my mind at the moment)?

Atually my host had 16 GB ram. I don't think I need more than 4 GB ram for a couple of years in my guests, but I am interested in knowing if there are performance difference between 32 and 64 guest VMs.

Regards, P.

Since every address needs to be specified with 64 bit there is a slightly memory overhead using 64 bit. Also address math can be done using 2^32 instead 2^64 wich in worst case could double the time (very unlikely though). Numerous studies has proofed that a host with no use of more than 4 GB of memory performs best in a 32 bit kernel and userland. If the same applies to a PAE kernel I have no idea.

 

[PaulVM]

Since every address needs to be specified with 64 bit there is a slightly memory overhead using 64 bit. Also address math can be done using 2^32 instead 2^64 wich in worst case could double the time (very unlikely though). Numerous studies has proofed that a host with no use of more than 4 GB of memory performs best in a 32 bit kernel and userland. If the same applies to a PAE kernel I have no idea.

Do you have any link?
Amount of difference in performance (1,10,100%)?
CPU/RAM/disks/net?
Compatibilty issue using additional software (zend, php modules, ...)?

Having no real needs for x64 at the moment, my interest is understand if starting now can be an investment for the future or if I can expect more trouble than the benefit of having a more scalable VM.

Can be interesting if someone that had widely tested x64 VM in production can give some feedback.

Thanks, P.

 

[mir]

Afaik:

Use 32bit if and ONLY if you have not enough RAM. Because 64bit needs more RAM and not enough RAM = massive performance issues.
You will have same or better performance with 64 bit as long as you have enough RAM (performance benefits depends on the application, will be something between 0% and 30%).

Can you define 'not enough RAM'? Is it amount of RAM in host or VM?

 

[PaulVM]

Afaik:

Use 32bit if and ONLY if you have not enough RAM. Because 64bit needs more RAM and not enough RAM = massive performance issues.
You will have same or better performance with 64 bit as long as you have enough RAM (performance benefits depends on the application, will be something between 0% and 30%).

So, do you say that generally x64 is better (if there is enough RAM).
My standard VMs have 0.5-1 GB (DNS,monitoring, list server, ...), 2 GB (standard httpd, mail proxy), 4 GB (medium httpd +MySQL, ...).
How much RAM is the minimun suggested for using x64?
In the past I had terrible experiencies with VmWare server were increasing RAM give decreasing of performance, so I am really careful in it ...

Thanks, P.

 

[sumsum]

I'm uslualy use 64bit guest systems (Centos 6.x) when they need 4GB or more RAM. No issues so far.

 

[JohnD]

Can you define 'not enough RAM'? Is it amount of RAM in host or VM?

Both is important.
You have not enough RAM as soon as your VM or your host starts to use SWAP (real use, low swap usage is normal) and you get I/O performance issues.

I know phoronix is not a good source but you will find much other sources on google. http://www.phoronix.com/scan.php?page=article&item=ubuntu_32_pae&num=1
A lot of applications (including apache) profits from 64bit so i would always choose 64bit as long as i have enough RAM (and i have no compatibility issues).