Re: spin_count and cpu usage

  • From: Stephane Faroult <sfaroult@xxxxxxxxxxxx>
  • To: ax.mount@xxxxxxxxx
  • Date: Mon, 04 Sep 2006 09:42:32 +0200

Alex,

I am afraid you have missed the "feng shui" part (which made me broadly smile) from Tim's answer. In plain English, playing with this type of parameter is very dangerous; I doubt that the Oracle support often recommends to do it (unless they are very desperate). It affects all your Oracle system and nobody can predict what the result will be. If you have latching issues (and you didn't say you had any), you will get much better results trying to understand why your applications are having this type of problem. Sometimes the solution is as simple as changing a job schedule. If you have big performance problems, look at the code that is running first.

Hope that helps

Stéphane Faroult
RoughSea Ltd


amonte wrote:
Hi Tim

Thanks very much foor the reply.

Since spin_count is just a numbe rof loops, if we have a faster CPU shouldnt we increase the value? I mean running 2000 loops in a 3GHZ CPU is definitely not the same as running in a 500MHz CPU. In fact i recall, Guy Harrison wrote a latch related paper mentioning that since 2000 for spin_count has been used since Oracle 7, that is 10 - 13 years ago a higher value for modern CPU wouldnt surprise.

Regarding the CPU usage, if with 8 500MHz CPU I consume 30000 seconds CPU in 1 hour, if I replace those with 1GHZ CPU would I reduce my CPU time/usage to half?


TIA

Alex



On 9/4/06, *Tim Gorman* <tim@xxxxxxxxx <mailto:tim@xxxxxxxxx>> wrote:

    Spin count has little to do with CPU cycles, and everything to do with
    the duration of non-pre-emptive waits on latches (i.e. column
    SPIN_GETS
    on V$LATCH).  It is literally the count in a loop that Oracle spins
    while waiting non-pre-emptively ( i.e. without losing the CPU) for a
    latch.  Really, really bad feng-shui to mess with spin count.

    Oracle cannot record if your CPU is working "efficiently" or not.  All
    it can do is report on how much CPU time was consumed.  If you do the
    math, 3 million centi-seconds is 30,000 seconds.  An hour has 3,600
    seconds, so eight CPUs can use 28,800 seconds in an hour.  It pretty
    much adds up to Oracle consuming all the CPU on your server, which
    is OK
    if it is the only thing on the server.  As far as the discrepency
    between 28,800 and 30,000, don't expect too much accuracy from a
    mechanism that is constantly rounding or truncating micro-seconds
    to the
    nearest centi-second...


amonte wrote: > Hi > > I was wondering if anyone know how is spin_count measured? In CPU CYCLES? > > I havea doubt with CPU usage as well, I undersstand that in v$sysstat > it is using centiseconds to measure CPU usage. I wonder how can we > determine 1 centisecond is equivalent to how many CPU cycles? For > example I have a server with 8 PA-RISC 1200 MHz CPU, in 1 hour it > reports 3 million centiseconds usage, am I using fully? Basically is > how do you know how efficiently is your CPU working from Oracle > Statistics. > > TIA > > Alex >

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