[HACKERS] possible effective_io_concurrency performance regression

-hackers,

While updating my Postgres performance curriculum I was doing some 
testing with effective_io_concurrency and I may have found a regression. 
I am aware that the parameter only works under certain conditions. 
However, what I appear to have found is that if it is set to anything 
but 0, it is a regression for (at least benchmarksql tpc-c) workloads.

See here:

Testing with the TPC style benchmark shows that on local systems, 
setting this between 0 – 48 keeps the TPS within noise level. However, 
testing also shows that on cloud systems such as Google Cloud Compute 
setting this setting to anything greater than 0 results in an 
approximately 10% performance degradation on TPS:

Local/GCE    effective_io_concurrency    TPS

GCE        OFF                47951
    8                43098
    1                43233
LOCAL    0                9939
    4                9960
    16                9955
    48                9958


I was able to produce these results pretty consistently. I wonder if any 
has seen this on EBS? GCE instance is 16CPU, 59GB memory, 240MB 
Sustained rate SSD with 15k IOPS.

Thanks,

JD

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