Re: What is Parallel DML?

This may depend to some degree on the nature and scale of the job. Parallel query often results in high I/O rates - if the CPU is going to be waiting for one process to complete it's I/O phase, it may as well do some work for another process.

I have found in the past that parallel query can scale quite well on suitable hardware on suitable types of query even when there is only one CPU.

On the other hand - when the two layers of slaves are busy flinging intermediate results all over the place, the cost of the task switching can become a significant fraction of the CPU used, and have a massive negative impact on performance - historically it used to be very common for Oracle to take paths which resulted in this sort of problem,

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Howard J. Rogers wrote in message <3b5bea5f_at_usenet.per.paradox.net.au>...


>

>That doesn't get around the fact that at any one moment, only a single

>process can be serviced by a single CPU, regardless of capacity.  OK, the

>cycles chewed up by the 'system idle time process' can be put to more

>effective use, but only one of those slave process can actually have time

 on


>the CPU at any given moment.  And *that's* the reason my query slowed down

>with a degree of parallelism of 2 -having to perform a context switch

>between two processes on the one CPU is expensive.

>

>Threads are a different game again, since there is no context switch when

>switching between threads within the one process.

>

>Your mileage may vary: mine never has.

>

>HJR