Tuesday, January 3, 2017

Oracle Database Cloud (DBaaS) Performance - Part 1 - CPU

After having looked at the performance consistency provided by the Oracle Database Cloud offering in the previous series, I'll focus here on the raw performance figures I've measured during my tests, starting with the CPU related performance findings.

One of the first surprises is related to the fact that Oracle uses a unit called "OCPU" to specify the CPU capacity provided, which is explained here:

So one "OCPU" is supposed to represent one core with two Hyperthreading CPU threads, and hence should correspond for example to two VCPUs used as unit by Amazon.

But when looking at the actual CPU configuration of such a DBaaS VM, I got the following results for a 4 OCPU configuration:

So, that 4 OCPU configuration provides 8 CPUs, which is expected, but it provides those 8 CPUs with one thread per core, so that means 8 cores.

This is what I get when I configure a corresponding Amazon EC2 VM with 8 VCPUs (m4.2xlarge), which should be same as the Amazon RDS "db.m4.2xlarge" configuration (but I can't access a RDS instance on O/S level, hence the EC2 fallback):

Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                8
On-line CPU(s) list:   0-7
Thread(s) per core:    2
Core(s) per socket:    4

Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 79
Model name:            Intel(R) Xeon(R) CPU E5-2686 v4 @ 2.30GHz
Stepping:              1
CPU MHz:               2300.062
BogoMIPS:              4600.12
Hypervisor vendor:     Xen
Virtualization type:   full
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              46080K
NUMA node0 CPU(s):     0-7

So this more in line to what is expected - 4 cores providing 8 CPU threads.

Does it make a difference in terms of actual performance? It does - when running my CPU tests, both the PL/SQL as well as the SQL engine based tests (see the previous "performance consistency" series for more details about the tests) show the following average duration per iteration per thread when running with 4 vs. 8 threads:

Oracle DBaaS:

 Amazon RDS:

So, using 8 threads instead of 4 threads only increases the duration of a test iteration slightly in the Oracle DBaaS 4 OCPU environment, whereas the Amazon RDS 8 VCPU environment significantly slows down, even more than expected considering Hyperthreading effects - in particular the SQL Logical I/O test requires more than twice of the time at 8 threads, but the PL/SQL based test, too, significantly slows down. It's interesting to see that running the SQL Logical I/O test at 4 threads the Amazon RDS environment outperforms the Oracle DBaaS offering to an unexpected degree.

Whether the better Amazon RDS SQL Logical I/O performance at 4 threads comes from the larger cache sizes reported by "lscpu" I don't know. I also don't know why Oracle provides more cores than outlined in their own product description. Maybe this should avoid exactly the effects seen with the Amazon RDS environment - maybe Hyperthreading doesn't work that well in virtualized environments - but that is just my speculation. Whether Oracle will keep this different approach in future I don't know either.

All I can say is that I consistently got that CPU / core ratio when configuring several services using a different number of OCPUs and that the my performance tests showed the difference outlined above when comparing the Oracle DBaaS and Amazon RDS environments.

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