By default, there is no affinity between CPUs and engines in Adaptive Server. You may see slight performance gains in high-throughput environments by establishing affinity of engines to CPUs.
Not all operating - systems support CPU affinity. The dbcc tune command is silently ignored on systems that do not support engine-to-CPU affinity. The dbcc tune command must be reissued each time Adaptive Server is restarted. Each time CPU affinity is turned on or off, Adaptive Server prints a message in the error log indicating the engine and CPU numbers affected:
Engine 1, cpu affinity set to cpu 4.
Engine 1, cpu affinity removed.
The syntax is:
dbcc tune(cpuaffinity, start_cpu [, on | off])
start_cpu specifies the CPU to which engine 0 is to be bound. Engine 1 is bound to the CPU numbered (start_cpu + 1). The formula for determining the binding for engine n is:
((start_cpu + n) % number_of_cpus
CPU numbers range from 0 through the number of CPUs minus 1.
On a four-CPU machine (with CPUs numbered 0–3) and a four-engine Adaptive Server, this command:
dbcc tune(cpuaffinity, 2, "on")
The command gives this result:
Engine |
CPU |
|
|---|---|---|
0 |
2 |
(the start_cpu number specified) |
1 |
3 |
|
2 |
0 |
|
3 |
1 |
On the same machine, with a three-engine Adaptive Server, the same command causes the following affinity:
Engine |
CPU |
|---|---|
0 |
2 |
1 |
3 |
2 |
0 |
In this example, CPU 1 is not used by Adaptive Server.
To disable CPU affinity, use -1 in place of start_cpu, and specify off for the setting:
dbcc tune(cpuaffinity, -1, "off")
You can enable CPU affinity without changing the value of start_cpu by using -1 and on for the setting:
dbcc tune(cpuaffinity, -1, "on")
The default value for start_cpu is 1 if CPU affinity has not been previously set.
To specify a new value of start_cpu without changing the on/off setting, use:
dbcc tune (cpuaffinity, start_cpu)
If CPU affinity is currently enabled, and the new start_cpu is different from its previous value, Adaptive Server changes the affinity for each engine.
If CPU affinity is off, Adaptive Server notes the new start_cpu value, and the new affinity takes effect the next time CPU affinity is turned on.
To see the current value and whether affinity is enabled, use:
dbcc tune(cpuaffinity, -1)
This command only prints current settings to the error log and does not change the affinity or the settings.
