Re: Turn off Hyperthreading! WAS: 60 core performance with 9.3

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От Graeme B. Bell
Тема Re: Turn off Hyperthreading! WAS: 60 core performance with 9.3
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Msg-id 5AECB9C6-298E-4B2F-9443-3DD6D1243EBA@skogoglandskap.no
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Ответ на Re: Turn off Hyperthreading! WAS: 60 core performance with 9.3  (Tatsuo Ishii <ishii@postgresql.org>)
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> HT off is common knowledge for better benchmarking result

It's wise to use the qualifer 'for better benchmarking results'.

It's worth keeping in mind here that a benchmark is not the same as normal production use.

For example, where I work we do lots of long-running queries in parallel over a big range of datasets rather than many
short-termtransactions as fast as possible. Our biggest DB server is also used for GDAL work and R at the same time*.
Prettyfar from pgbench; not everyone is constrained by locks. 

I suppose that if your code is basically N copies of the same function, hyper-threading isn't likely to help much
becauseit was introduced to allow different parts of the processor to be used in parallel when you're running
hetarogenouscode.  

But if you're hammering just one part of the CPU... well, adding another layer of logical complexity for your CPU to
manageprobably isn't going to do much good. 

Should HT be on or off when you're running 64 very mixed types of long-term queries which involve variously either
heavyuse of real number calculations or e.g. logic/string handling, and different data sets? It's a much more complex
questionthan simply maxing out your pgbench scores.  

I don't have the data now unfortunately, but I remember seeing a benefit for HT on our 4 core e3 when running
GDAL/Postgiswork in parallel last year. It's not surprising though; the GDAL calls are almost certainly using different
functionsof the processor compared to postgres and there should be very little lock contention. In light of this
interestingdata I'm now leaning towards proposing HT off for our mapservers (which receive short, similar requests over
andover), but for the hetaragenous servers, I think I'll keep it on for now. 

Graeme.



* unrelated. There's also huge advantages for us in keeping these different programs running on the same machine since
wefound we can get much better transfer rates through unix sockets than with TCP over the network. 

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