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Comparison of scaling behaviour

When we look at the scaling behaviour of the two servers, we see limitations crop up in certain places: the step from one to two processors is less smooth for each of the three databases than what we witnessed before. Even though the results are impressive in an absolute sense, we see, for instance, that adding a second Woodcrest in MySQL 5.0.20a – costing a good 851 dollars – only yields a 6% performance increase. Although it is not the case that this version scales well on other platforms, we do experience this as a disappointment.

The unsatisfactory scaling behaviour of the faster chip cannot simply be attributed to a lack of bandwidth: clock speed – and hence the theoretical computational power of the processor – has increased by 12.5%, while the memory supply an extra band width of 25% in theory. On paper, this means that an extra ten percent more bytes should be available at each tick of the clock for the processors in the Dell PowerEdge than for the Fujitsu RX300's CPU's. Latency, however, has increased: one system has 533MHz of CL4 memory, while the other runs on 667MHz CL5. It is no coincidence that this can be re-valued at 7.5ns in both cases. Since the processor measures time in clock ticks, the same latency gets comparatively longer as the frequency increases. A 2.66GHz chip will see 20 ticks go by in these 7.5ns, while a 3.0Ghz version ticks 22.5 times – a 12.5% increase.

An additional factor that can contribute to poor scaling behaviour is a heavier loading of the buses. In theory, the double 1333MHz FSB has the same band width as the four memory channels, but leaves no headroom for internal communication between the processors. Doing more work increases the need to communicate, leaving less room for reading and writing data. Of course, the latency story is also applicable to the communication between the processors. A final factor that may contribute to the situation is that the minimal amount of I/O actions increases by having faster throughput.

Woodcrest 2.66GHz / 533MHz FBD
Performance scaling
MySQL 4.1.20 | 1x single -> 1x dual 34%
MySQL 4.1.20 | 1x dual -> 2x dual 17%
MySQL 4.1.20 | 1x single -> 2x dual 56%
MySQL 5.0.20a | 1x single -> 1x dual 22%
MySQL 5.0.20a | 1x dual -> 2x dual 14%
MySQL 5.0.20a | 1x single -> 2x dual 40%
PostgreSQL 8.2-dev | 1x single -> 1x dual 76%
PostgreSQL 8.2-dev | 1x dual -> 2x dual 84%
PostgreSQL 8.2-dev | 1x single -> 2x dual 224%
Woodcrest 3.0GHz / 667MHz FBD
Performance scaling
MySQL 4.1.20 | 1x single -> 1x dual 34%
MySQL 4.1.20 | 1x dual -> 2x dual 13%
MySQL 4.1.20 | 1x single -> 2x dual 52%
MySQL 5.0.20a | 1x single -> 1x dual 29%
MySQL 5.0.20a | 1x dual -> 2x dual 6%
MySQL 5.0.20a | 1x single -> 2x dual 37%
PostgreSQL 8.2-dev | 1x single -> 1x dual 84%
PostgreSQL 8.2-dev | 1x dual -> 2x dual 77%
PostgreSQL 8.2-dev | 1x single -> 2x dual 226%



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