1. Should I have at least one logical processor per instance, given that I currently only have 4 logical processors for 6 instances and MAX_DOP set to 0?
That depends, how much CPU usage is each instance using on average? You can get this information from the default health session extended event that is running (assuming 2008+).
Four logical processors may be perfectly adequate for this workload - we won't know until we have data. Having said that, since each instance of SQL Server acts on its' own and has no idea of the other instances installed, I'm sure Windows isn't too happy about swapping threads.
I'd take a look at my wait stats dmv and see if we have a higher percentage of wait time on the signal_wait part which would indicate scheduling issues and possible contention between instances. Additionally I'd trend this with the cpu context switches to see if there is a weak or direct correlation with "bad times" and "good times" of instance health.
My gut, though, tells me that unless these are woefully underutilized instances that this server is bound for issues - whether they are happening now or in a few months from now.
2. If I had one logical processor per instance, should I leave MAX_DOP at 0 or limit each instance to MAX_DOP = 1?
MAXDOP just limits the number of logical processors ('schedulers') that a single parallel query may use during execution. There is nothing stopping SQL Server from running multiple parallel queries; in fact I've diagnosed and fixed this very issue a multitude of times for places that didn't even know they had a scheduling problem (it was looked at as a "blocking" problem).
Setting MAXDOP to 1 essentially makes all user queries single threaded. This, again, doesn't stop SQL Server from executing more than a single task at a time as it merely forces serial executions. This means each instance has four (4) visible online schedulers for a total of 4*x number of instances possible concurrent queries. Setting MAXDOP here doesn't solve the issue of being logically overburdened, not taking into account the actual setting of the virtual server as to whether or not it can run on hyper threaded (if enabled) cores. Yikes.
I'm not saying forcing MAXDOP to 1 is good or bad, just that we have no data to go on. Thus we won't know the impact of the setting. Again, though, I'd DEFINITELY not have it set to four (4)!
3. Seeing as it is a Standard Edition should I limit MAX_DOP to 4 as an alternative?
Well, it can't be higher than four (4), so it's already limited. This question, I believe, is closely coupled with the question above. Hopefully I've explained it to a satisfactory level of understanding.
If you want to get an idea (due to the small threshold for parallelism) of how many queries are being parallelized, you can check the execution plans dmv. This is a fairly expensive operation, so please do it off hours or times of little to no usage between ALL of the instances. Remember, what you do to one instance is going to be (roughly) done to all because of the shared nature of the server.
Best Answer
The idea of having multiple instances on one server can have various reasons.
1. SQL Server License Costs
You can have multiple instances all running on one (big?) server and not have to pay large amounts of licensing fees.
2. Upgrades / Service Packs / Cummulative Updtes / Hotfixes
Having multiple instances on one (big?) server, allows you to patch the individual instances to different levels. You can have once instance running Service Pack 1 and another instance running Service Pack 2. And on another instance you could even test drive the hotfix you're not sure it will work well.
3. Permissions / SQL Server Roles / ....
Having multiple instances allows you to assign the
sysadmin
server role to individuals, without giving them the ability to destroy all the databases if they would be located on one instance. You have a separation between databases of one customer and another customer.4. Windows Server Licenses
You can save on Windows Server licenses, because you only require one server license for multiple SQL Server instances.
5. Individual IP Addresses for Each SQL Server Instance
You can setup the individual SQL SErver instances to only respond to one IP address which in turn is linked to a CNAME/ALIAS. e.g. you could have your Windows Server running on 10.0.0.1 and each of your SQL Server instances running on 10.0.0.2 ... 10.0.0.5 which in turn would have their own CNAME:
Each instance is then only reachable via the IP address and Port 1433 which in turn allows for IP to IP firewall configuration.
See my question Configure SQL Server Analysis Services (SSAS) to run on same IP as SQL Server instance for an example of how you could configure multiple instances. Or the question Multi-instance SQL Server Standard Editon MaxDop settings where I show how we setup multiple insances at our shop.