How fast is fast enough?

Occasionally I’m asked to comment on a cooltst analysis of workload suitability for UltraSPARC T1 and T2 systems, and I can’t draw a conclusion because I don’t know the customer’s intentions. An example: I saw one process using 11.6% of a CPU. If a single thread were consuming that much CPU I’d hesitate to recommend the workload for T1 or T2 processors because I might expect the workload to keep only 9 threads busy (100% / 11.6%). Since a T1 processor has 32 hardware threads and a T2 processor has 64, most of the throughput potential of the system could be wasted.

However, the process that was using 11.6% had 546 threads. If the CPU is spread at all evenly across those threads, then there is ample parallelism to make full use of a T1 or T2 processor. You need to dig down into the LWP details of the cooltst report to see what’s going on. In a future relase of cooltst, that part of the report will get a little easier to read.

But what if a single thread were using 11.6%? Would that automatically disqualify the workload? Not necessarily. In this case the workload was running on a 4 processor 1.06 GHz UltraSPARC-IIIi system. If the customer was meeting his quality of service requirements using only one ninth of one of those relatively slow processors, then a T1 or T2 processor ought to be able to meet or exceed those requirements, regardless of how much excess capacity of the system went unused.

So what is the customer’s intention? Does he want dramatically improved response time over his current system, or to maintain current service levels? Will the workload remain the same, or grow? Will it grow by adding more work to a single thread of computation, or will there be more users doing the same thing? Will he be consolidating the workloads of several current systems onto virtual machines on a single Sun SPARC Enterprise T5220? Despite the workload measurements shown by cooltst, there could be ample parallelism to exploit the throughput potential of UltraSPARC T1 and T2.


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