Re: Questions about checksum feature in 9.3

Ants Aasma-2 wrote
>> So, has anyone compiled checksum vectorized on OS X? Are there any
>> performance data that would indicate whether or not I should worry with
>> this in the first place?
>
> Even without vectorization the worst case performance hit is about
> 20%. This is for a workload that is fully bottlenecked on swapping
> pages in between shared buffers and OS cache. In real world cases it's
> hard to imagine it having any measurable effect. A single core can
> checksum several gigabytes per second of I/O without vectorization,
> and about 30GB/s with vectorization.

Thoughts on how/where to provide guidance as to this kind of concern. The
single paragraph in the initdb documentation seems to be lacking. Would a
destination page on the wiki, linked to from the documentation, where
"current knowledge" regarding benchmarks and caveats can be stored, be
appropriate.

To that end, Ants, do you actually have some resources and/or benchmarks
which support your claim and that you can provide links to?

The "single" core aspect is interesting. Does the implementation have a
dedicated core to perform these calculations or must the same thread that
handles the relevant query perform this work as well? How much additional
impact/overhead does having to multitask have on the maximum throughput of a
single core in processing checksums?

This whole vectorization angle also doesn't seem to be in the
documentation...though I didn't look super hard.

David J.

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