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Re: Using complete CPU core instead thread

Posted By: Martin Furderer
Date: Tuesday, 20 February 2018, at 10:59 a.m.

In Response To: Re: Using complete CPU core instead thread (R MICHAEL)


I'm not sure I completely understood the original question.

Anyway, here's some (not too much in-depth) explanation that may be helpful:

Informix has a homegrown implementation of multi threading. (This is mainly because at the time the Posix thread implementation first was not and then not widely available on the many different platforms where Informix was running (Sequent, Iris, Data General Unix, SINIX, to name a few). Furthermore, it is also of some advantage that this homegrown multi threading is non-preemptive as this allows thread switches at times when they are opportune and less costly.)

An Informix Virtual Processor (VP) is a UNIX/Linux process (of the oninit binary) that normally has several threads, but runs only one of these threads at a time. Therefore, a single VP will utilize only a single CPU at any one time. If there are more CPUs available in the system, then the way to make Informix utilize them is to start more VPs, especially the so-called CPU VPs as they are doing most of the normal work by running the so-called "sqlexec" threads. With the threads running in a VP being of the homegrown multi threading implementation, the OS does not really know about them and therefore does not schedule them concurrently. The OS only knows about the VPs (that are processes) and schedules those.

Bringing in the concept of Intel HyperThreading (and very similar also Simultaneous Multi Threading - SMT - on Power) does not change anything of the above. With HyperThreading enabled, the OS basically sees two CPUs for every processor core. Therefore it can schedule twice as many processes concurrently. In order to fully take advantage of this, one would have to re-configure Informix to run twice as many processes (i.e. VPs).

Now, thing with HyperThreading (as well as SMT) is, that - obviously - it does not magically create twice as many "real CPUs". What it does is to offer the OS the possibility to schedule twice as many processes concurrently ... and then runs certain operations executed by two concurrent processes in parallel, taking advantage of arcane details of the processor and system architecture (instruction execution pipeline, etc.). Therefore, the two processes that are seemingly running concurrently on the two CPUs that the HyperThreading offers for the same core ... only run partially in parallel. How well this pseudo-parallelism works out (i.e. how much performance improvement it creates) depends a lot on how the application is implemented and therefore normally cannot be predicted with much accuracy. With an application like the Informix database server it may even considerably depend on the database applications.

With all this, it hopefully becomes a bit clearer that and why:

- a single Informix VP does not utilize two CPUs concurrently as it is a single process from the perspective of the OS.

- the Informix affinity configuration cannot make a single VP to utilize two CPUs concurrently. (Binding a VP to one or several CPUs basically tells the OS to restrict the scheduling of the process to the specified CPUs.)

- switching on HyperThreading does not automatically double the performance.

- with HyperThreading active, Informix probably should be re-configured to run more (CPU) VPs in order to take advantage of the HyperThreading.

- doubling the number of Informix VPs for HyperThreading probably is not the ideal configuration because the processes only run partially in parallel. Doubling the number of VPs may well overload the system and thus cause some negative effect on performance.

- the ideal Informix configuration for a system with HyperThreading should be determined by careful tuning of the individual system using the intended workload.

Regards, Martin
Martin Fuerderer
Informix Development Germany


HCL Technologies Germany GmbH
Frankfurter Ring 17
80807 Munich, Germany

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From: <> on behalf of R MICHAEL <>
Sent: Tuesday, February 20, 2018 14:58
Subject: Re: Using complete CPU core instead thread [40717]

Hi Benjamin,

I forgot those important details:
I use Linux on Intel.
Unfortunately I don't have where to test so I tried to ask here

Thank you.

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