Interesting discussion. I don't agree with a lot of things said here, mostly about hyperthreading and "overloading" cores with Informix CPUVPs, but, whatever works for some (test) environments has a right to be presented, I guess.
Some AIX / POWER comments.
On POWER you are very likely to become familiar with NUMA (Non Uniform Memory Architecture). It's not something specific to this hardware platform, but higher likelihood is from the fact that POWER usually has less cores per CPU (socket or node) compared to AMD/Intel and even SPARC.
In that environment (and for certain types of workloads) it's frequently not about how many cores you give to Informix, but about where these cores (and memory) are coming from. Make sure you become familiar with output from "lssrad -av" and understand "affinity domains", and most importantly, never run Informix on CPUs and memory coming from different CECs, no good will come out of it.
Next thing to consider is the optimization goal - namely "throughput" or "response time". For the latter you'll want lower SMT, but for the throughput - everything will be fine up to SMT-8 (unless you run into NUMA)
The "sweet spot" for Informix is between one and two
active sessions per CPUVP (best efficiency), but Informix on POWER handles higher load quite well if you run Informix on resources with maximum degree of affinity.
One more thing to consider is the I/O to CPU ratio. For most deployments on the current hardware CPU capability makes it very likely to hit I/O limit for the storage subsystem. You must make every effort to fit the "working set" in memory (Informix buffer pool). If that's not possible - the flash storage (or any storage that can provide sustained 200k IOPs or better) is the only answer.
In terms of testing methodology, if it comes to that, there is noticeable difference in running Informix and "client" on the same cores vs them using separate resources. Get familiar with "rsets" and very useful commands like "execrset", etc.
It is recommended running the instance in dedicated capped LPAR using resources with maximum possible affinity for consistent and predictable performance.
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Vladimir Kolobrodov
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Original Message:
Sent: Mon November 29, 2021 05:08 AM
From: John Smith
Subject: How many aio/KAIO
Thanks for your useful post, i noted this on Linux, but for this moment, i'm interested of IBM Power with AIX ;), and unfortunately did"t have to means to do benchmarks, so i'll be thankful of experiences on Power Systems with AIX
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John Smith
Original Message:
Sent: Mon November 29, 2021 04:42 AM
From: Benjamin Thompson
Subject: How many aio/KAIO
As Art has already indicated in his post, the answer to this may be platform dependent.
I have often wondered about the answers to questions like this and am fortunate that our company has invested in a very nice application load testing set up. With this we've provided HCL with some test results which have fed into 14.10.FC6 and FC7 performance enhancements. These versions are much faster in a handful of areas, notably ER in 14.10.FC6+. It's been years since I have been near a non-Linux/Intel system though.
In our load testing on Linux, we do not see any benefit from increasing VPCLASS CPU beyond the CPU core count, i.e. having multiple CPU VPs per core. Having even two per core may even be marginally detrimental, although our testing finds it doesn't make a lot of difference. I agree about hyperthreading, although we don't switch it off, we use the processor affinity setting to ensure one CPU VP per physical core.
On Art's statement about a maximum 500-750 MHz of processing cycles per CPU VP, it is not something I observe on our kit. I can only speculate this may be something to do with the bus speed relative to the processor speed on some hardware, or a lack of NUMA.
Again on Linux if you have a lot of CPUs VPs especially, you should increase kernel parameter aio-max-nr to 2, 4 or even 8 million. This can restrict throughput and there is no metric that exposes I/O requests blocked by this system limit, beyond the raw numbers in your test.
Ben.
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Benjamin Thompson
Original Message:
Sent: Fri November 26, 2021 03:19 AM
From: John Smith
Subject: How many aio/KAIO
Hello All
When using KAIO, what is recommended for vpclass CPU and AIO please ?
Thanks
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John Smith
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#Informix