Re: high user cpu, massive SELECTs, no io waiting problem

Yeah, at max load we are. We're running quad 12 core AMD Magny Cours.
Under max load all of our cores go about 20 to 30% red (i.e. kernel)
and we wind up waiting on the kernel much more. Could be a mix of
context switching and waiting on memory, so it's just a guesstimate
I'm making based on previous testing with Greg Smith's memory
streaming test and familiarity with this system.

On Wed, Feb 16, 2011 at 8:53 AM, Strange, John W
<john(dot)w(dot)strange(at)jpmchase(dot)com> wrote:
> Scott, are you really moving that much data through memory, 70-80GB/sec is the limit of the new intel 7500 series in a best case scenario.
>
> - John
>
> -----Original Message-----
> From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Scott Marlowe
> Sent: 16 February 2011 15:43
> To: Marti Raudsepp
> Cc: Thomas Pöhler; pgsql-performance(at)postgresql(dot)org; Felix Feinhals; Verteiler_A-Team; Björn Metzdorf
> Subject: Re: [PERFORM] high user cpu, massive SELECTs, no io waiting problem
>
> On Wed, Feb 16, 2011 at 6:44 AM, Marti Raudsepp <marti(at)juffo(dot)org> wrote:
>> On Tue, Feb 15, 2011 at 20:01, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> wrote:
>>> run htop and look for red.  if youi've got lots of red bar on each CPU
>>> but no io wait then it's waiting for memory access.
>>
>> I don't think this is true. AFAICT the red bar refers to "system
>> time", time that's spent in the kernel -- either in syscalls or kernel
>> background threads.
>
> My point being that if you've got a lot of RED it'll be the OS waiting
> for memory access.  Trust me, when we start to hit our memory
> bandwidth (in the 70 to 80 GB/s range) we start to get more and more
> red and more and more kernel wait time.
>
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