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Re: Hardware/OS recommendations for large databases (

From: Alan Stange <stange(at)rentec(dot)com>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Luke Lonergan <llonergan(at)greenplum(dot)com>, Dave Cramer <pg(at)fastcrypt(dot)com>, Joshua Marsh <icub3d(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Hardware/OS recommendations for large databases (
Date: 2005-11-20 18:09:06
Message-ID: 4380BBC2.7060109@rentec.com (view raw or flat)
Thread:
Lists: pgsql-performance
Greg Stark wrote:
> Alan Stange <stange(at)rentec(dot)com> writes:
>
>   
>>> Iowait is time spent waiting on blocking io calls.  As another poster
>>> pointed out, you have a two CPU system, and during your scan, as predicted,
>>> one CPU went 100% busy on the seq scan.  During iowait periods, the CPU can
>>> be context switched to other users, but as I pointed out earlier, that's not
>>> useful for getting response on decision support queries.
>>>       
>
> I don't think that's true. If the syscall was preemptable then it wouldn't
> show up under "iowait", but rather "idle". The time spent in iowait is time in
> uninterruptable sleeps where no other process can be scheduled.
>   
That would be wrong.   The time spent in iowait is idle time.  The 
iowait stat would be 0 on a machine with a compute bound runnable 
process available for each cpu.

Come on people, read the man page or look at the source code.   Just 
stop making stuff up.


>   
>> iowait time is idle time. Period.   This point has been debated endlessly for
>> Solaris and other OS's as well.
>>
>> Here's the man page:
>>               %iowait
>>                      Show  the  percentage  of  time that the CPU or CPUs were
>>                      idle during which the system had an outstanding disk  I/O
>>                      request.
>>
>> If the system had some other cpu bound work to perform you wouldn't ever see
>> any iowait time.  Anyone claiming the cpu was 100% busy on the sequential scan
>> using the one set of numbers I posted is misunderstanding the actual metrics.
>>     
>
> That's easy to test. rerun the test with another process running a simple C
> program like "main() {while(1);}" (or two invocations of that on your system
> because of the extra processor). I bet you'll see about half the percentage of
> iowait because postres will get half as much opportunity to schedule i/o. If
> what you are saying were true then you should get 0% iowait.
Yes, I did this once about 10 years ago.   But instead of saying "I bet" 
and guessing at the result, you should try it yourself.     Without 
guessing, I can tell you that the iowait time will go to 0%.  You can do 
this loop in the shell, so there's no code to write.  Also, it helps to 
do this with the shell running at a lower priority.

-- Alan



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