Process runs faster when executed independently, but runs slower when a parallel big process is running

Question

Hi,
The issue is,  a process, say "A" is running fine on certain days. On some days there are additional processes running on the server. When these additional processes are running, "Process A" suffers performance issue.
Interesting point is that, these additional processes were running since long time. But, earlier "Process A" was running fine even when these additional process were running. Suddenly from past 2 or 3 weeks there is performance issue in "Process
A", when these additional process run.
Note: Nothing has been modified for the process A.
Process A is an SQL job which has SSIS Package and stored proc in different steps
When multiple parallel processes are running, SSIS Package step suffers around 40% increase in execution time, where as stored procs have only 15-20% increase in execution time
When Process A is executed while no other big processes are running, it's execution time is fine. From past few days, issue is only when, some other big parallel processes are running.
Currently below is my analysis:
Since Process A is running fine when it is executed independently, I assume there is no issue in this process.
Since issue occurs when some other big process is running, when Process A is running, I believe it is DiskIO issue. Will the issue be resolved if RAM size is increased?
Is there any way to check if RAM is being fully utilized by the server
Is there any other possibility why there is a sudden dip in performance when parallel processes are running.
Is there any possibility of having issues in additional processes. Incase if it has some issues, does it impact "Process A"
Please let me know if you need any further information. Infact I am not able to diagnose what is the actual root cause for the performance issue in "Process A" as nothing has been modified.
Also it would be very much helpful, if I get any idea on different ways in reaching the actual root cause of this performance issue.
NOTE: This is a data warehouse
Thanks,
Raksha

Answer 1

When a query has a parallel plan, it will in general try to grab all cores up to the maxdop setting, but then it often uses them inefficiently. 
But in that case, there are better odds for the queries not battling on resources with each other!
What Josh alludes is to the fact that SQL Server needs to partition the data, so that different partitions are processed on different cores. This partitioning is based on statistics, and the statistics may be out of date or not accurate enough. This may
result in that the data is not partitioned proportionally, and some threads gets very little data to work with. Thus, these CPUs are still idle, so there may still be room for the two processes to run at full speed. (That is, full speed with the given plan,
which is not the full speed, had the partitioning been accurate.)
I mention this because you asked about parallelism, and many systems leave maxdop at its default setting of 0 (which means "go ahead and grab everything!") even though Microsoft recommends you set it to a different number depending on this and
that.
Since this is a data warehouse, Raksha should not tamper with "max degrees of parallelism", I think.
Erland Sommarskog, SQL Server MVP, [email protected]