IDS Forum

Re: AW: processor affinity

Posted By: Martin Fuerderer
Date: Thursday, 1 December 2005, at 9:03 a.m.

In Response To: Re: processor affinity (Martin Fuerderer )

Hi,

no, I don't agree.

The problems are:

a) Overestimated or not depends on the expectations.
With right expectations it will not be overestimated.

b) It is a statement that is too general.
Effects depend on individual facts in a specific scenario.
E.g. one more factor (of many) influencing performance
gain is how busy the CPUVPs are.

c) As I tried to point out, it is difficult to predict performance
benefits. However, that is the case with other features as
well.

d) Certainly it is not a panacea. It is not like "You have
performance problems ? Just use affinity and you'll be fine."

e) A dedicated machine for IDS probably is not the scenario
where affinity will bring most performance gain. That scenario
probably is where lots of other things are also running (e.g.
application). In such a scenario, there will be a much higher
competition for CPU. Therefore there will be much more
CPU context switches, and therefore I would expect a better
return from affinity - for IDS itself.
Whether that results in higher overall system performance
again depends on other factors, e.g. how the other
applications (running on the same machine) are
implemented.

As with all performance questions/problems the right thing
to do probably is to first determine, where the bottle neck
is. Then fix that in way without causing a new bottle neck
elsewhere, with possibly even worse overall effects.

Regards,
Martin
--
Martin Fuerderer
IBM Informix Development Munich, Germany
Information Management

"Andreas.KUTSCHE@spar.at" <andreas.kutsche@spar.at> wrote on 01.12.2005
12:28:04:
> Hello,
>
> would you agree that processor affinity is overestimated?
>
> I think there was the recommendation of configuring more CPUVPs than
> physical CPUs (on machines with fast processors).
> That seems a little bit contradictionary to the reasons for using
> CPU affinity, isn't it?
>
> Even if you have a dedicated database server I'm not sure if
> CPU affinity really helps to improve performance - has anyone
> seen that in real life?
> For mixed systems (multiple IDS instances or IDS and applications)
> I think CPU affinity not only doesn't help but it will even lower
> the overall performance.
>
> I would like to hear about real life installations that prove
> me wrong.
>
> Regards,
> Andreas
>
> -------------------------------------------
> SPAR Oesterreichische Warenhandels-AG
> Hauptzentrale
> Europastrasse 3
> A - 5015 Salzburg
>
> Tel: +43 662 4470 24223
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> -------------------------------------------
> -----Urspr|ngliche Nachricht-----
>
> > Von: forum.subscriber@iiug.org [mailto:forum.subscriber@iiug.org]Im
> > Auftrag von Martin Fuer....
> > Gesendet: Donnerstag, 01. Dezember 2005 09:50
> > An: ids@iiug.org
> > Betreff: Re: processor affinity [6056]
> >
> > Hi,
> >
> > you can still see performance benefits, at least in theory.
> >
> > [ In general even in a machine with several CPUs you will
> > have more processes running than there are CPUs. Therefore
> > at some time processes have to give up CPU and let others
> > run. When a particular process then gets the next turn to run
> > again, this can be on a different CPU. All this may depend
> > on the scheduling model that the OS uses (or that you have
> > configured the OS to use).
> >
> > With such a switch of CPU, there are a couple of things involved.
> > E.g. the new CPU's cache may need to be filled with the
> > processes context, etc. and the old CPU's cache probably needs
> > to be filled with a different processes context. This is additional
> > work to be done, often referred to as context switch.
> >
> > The idea of affinity is to avoid such additional work by making
> > sure that a certain process always runs on the same CPU. I.e.
> > it will not migrate from one CPU to another CPU upon being
> > scheduled for runtime. Ideally the process will always be able
> > to run on that CPU, i.e. the CPU is dedicated for this sole
> > process. ]
> >
> > Whether all this works out to a noticable performance benefit
> > depends on some other factors. How many CPUs does the
> > machine have vs. how many processes are running. How many
> > process switches are actually happening, which is influenced
> > by even other factors. As mentioned before, the actual
> > scheduling model/method used by the OS also plays a role
> > in this.
> >
> > With that I guess in practice you will only find out by trying/testing
> > in your specific environment. With several CPUs and several
> > CPUVPs configured in IDS, there will be several permutations
> > of affinity configuration that will be of interest for testing.
> >
> > Regards,
> > Martin
> > --
> > Martin Fuerderer
> > IBM Informix Development Munich, Germany
> > Information Management
> >
> > forum.subscriber@iiug.org wrote on 30.11.2005 21:27:04:
> > > On the topic of affinity, for a dedicated Solaris box with one
> > > install of IDS is processor affinity a good idea?
> > >
> > > Zev Berezin
> > >
> > > Date sent: Wed, 30 Nov 2005 13:20:02 -0500 (EST)
> > > To: ids@iiug.org
> > > From: "ART KAGEL, ...." <KAGEL@bloomberg.net>
> > > Subject: Re: processor affinity [6050]
> > >
> > > > Yes you certainly can. However, take a look at likely
> > usage patterns
> > for the
> > > > two instances. If their primary peak periods are
> > disjoint from one
> > another,
> > > > then you may do better to let them share more processors than to
> > restrict each
> > > > instance to a smaller subset of processors. You could still use
> > affinity to
> > > > keep the servers to different processors than other applications
> > though.
> > > >
> > > > Art S. Kagel
> > > > ----- Original Message -----
> > > > From: Bob Roussey <robert.roussey@spiritair.com>
> > > > At: 11/30 12:39
> > > >
> > > > I am running two instances on a QA server. Can I use processor
> > affinity for
> > > > both instances if they copy my production instance?
> > Production has 8
> > processors
> > > > and 6 virt cpus.
> > > >
> > > >
> > > > RedHat AS3 Linux, IDS 10.00.UC3
> > > >
> > > >
> > > > Bob Roussey
> > > > robert.roussey@spiritair.com

Messages In This Thread

• processor affinity
  Bob Roussey -- Wednesday, 30 November 2005, at 12:14 p.m.
  • Re: processor affinity
    ART KAGEL, BLOOMBERG/ 731 LEXIN -- Wednesday, 30 November 2005, at 1:19 p.m.
    • Re: processor affinity
      Zev Berezin -- Wednesday, 30 November 2005, at 3:27 p.m.
      • Re: processor affinity
        Martin Fuerderer -- Thursday, 1 December 2005, at 3:49 a.m.
        • AW: processor affinity
          Andreas.KUTSCHE@spar.at -- Thursday, 1 December 2005, at 6:35 a.m.
        • Re: AW: processor affinity
          Martin Fuerderer -- Thursday, 1 December 2005, at 9:03 a.m.

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