Investigating Cavium's ThunderX: The First ARM Server SoC With Ambition

Single-Threaded Integer Performance: SPEC CPU2006

Even though SPEC CPU2006 is more HPC and workstation oriented, it contains a good variety of integer workloads. Running SPEC CPU2006 is a good way to evaluate single threaded (or core) performance. The main problem is that the results submitted are "overengineered" and it is very hard to make any fair comparisons.

So we wanted to keep the settings as "real world" as possible. We welcome constructive criticism to reach that goal. So we used:

• 64 bit gcc: most used compiler on Linux, good all round compiler that does not try to "break" benchmarks (libquantum...)
• -Ofast: compiler optimization that many developers may use
• -fno-strict-aliasing: necessary to compile some of the subtests
• base run: every subtest is compiled in the same way.

The ultimate objective is to measure performance in applications where for some reason – as is frequently the case – a "multi thread unfriendly" task keeps us waiting.

Nobody expect the ThunderX to be a single threaded performance wonder. Cavium clearly stated that they deliberately went for a high core count with pretty simple cores. As a result, single threaded performance was not a priority.

However, Facebook and other hyperscalers have indicated that they definitely prefer to get the single threaded performance of a Xeon D. So any competitor challenging Intel should try to keep up with the Xeon D in single threaded performance and offer a throughput-per-dollar/watt bonus. So it is very interesting to measure what single threaded performance the current ThunderX can offer.

Although some of you have a mathematical mind and are able to easily decipher these kinds of tables, let the rest of us be lazy and translate this into percentages. We make the Xeon D-1581 the baseline. The Xeon D-1557's performance is more or less the single threaded performance some of the important customers such as Facebook like to have.

First of all, single threaded is somewhat better than we expected when we received the first architectural details (a very simple dual issue core with high latency shared L2). However, this is still a fraction of the Xeon D's single threaded performance, which means that ThunderX doesn't look very impressive to companies which feel that single threaded performance should not be lower than a low end Xeon D. The latter is 2 to 4 times faster. On average, the Xeon D-1581 delivers 3 times faster single threaded performance than the ThunderX, but not 5!

SPEC CPU2006 allows us to characterize the ThunderX core a bit better. We ignore libquantum because it has a very special profile: you can triple the score with specific compiler settings, but those settings reduce performance by 2-30%(!) in some other subtests. Those compiler settings optimize cache utilization by splitting records of an array in separate arrays. Combine this with software loop prefetching and libquantum numbers can indeed double or triple. Since libquantum is hardly relevant for the server world and is known for being a target for all kind of benchmark trickery, we ignore it in our comparison.

Mcf exhibits a large amount of data cache misses and memory controller usage. Mcf is also "horribly low IPC" software, so beefy execution backends do not help. Despite those facts, the ThunderX does not do well in mcf. Mcf does a lot of pointer chasing, so the high latency L2-cache and the high latency DRAM access are slowing things down. That is probably also true for XML processing and the network simulator: those subtests have the highest data cache misses.

The shallow pipeline and relatively powerful gshare branch predictor make the ThunderX a better than expected performer in the chess (sjeng), pathfinding (astar), compiling (gcc) and AI (gobmk). Although the gobmk has a relatively high branch misprediction rate on a gshare branch predictor (the highest of all subtests), the ThunderX core can recover very quickly thanks to its 9 stage pipeline. Notice also that gobmk and gcc have relatively large instruction footprints, which gives the ThunderX and its 78 KB I-cache an advantage.

That is also true for the perl, but that benchmark has a relatively high IPC and needs a beefier execution backend. Indeed, the more compute intensive (and thus high IPC sub tests) perlbench and hmmer perform badly relative to the Intel core. In these benchmarks, the wide architecture of the Intel cores pays off.