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Re: spinlocks on HP-UX

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: spinlocks on HP-UX
Date: 2011-08-30 04:59:01
Message-ID: 7674.1314680341@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Ants Aasma <ants(dot)aasma(at)eesti(dot)ee> writes:
> On Mon, Aug 29, 2011 at 10:12 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Also, if the PPC machine really is hyperthreaded (the internal webpage
>> for it says "Hyper? True" but /proc/cpuinfo doesn't provide any clear
>> indications), that might mean it's not going to scale too well past 16x
>> the single-thread case.

> According to IBM docs [1], 8406-71Y contains one 8 core POWER7 chip
> that is 4-way multi-threaded and has 4 memory channels.

Yeah, I looked at the docs.  "Multi threading" is IBM's term for
hyperthreading, that is several instruction streams competing for use of
a single processor core's pipelines.  So the 32 virtual processors on
the machine only really represent 8 physically independent cores,
squaring with the hardware designation.  I found an IBM doc
http://www-03.ibm.com/systems/resources/pwrsysperf_SMT4OnP7.pdf
suggesting that the throughput benefit of 4-way SMT is typically 1.5 to
2X, that is you max out at 1.5 to 2X as much work as you'd get with just
8 virtual processors on the same 8 cores.  So I'd say we're really doing
quite well to get the numbers I got.  (The paper also implies that you
get more benefit from SMT with workloads that incur more memory-access
stalls, so the relatively large working set of this test case is helping
it look good.)

> 6GB of
> memory is a strange amount for the IBM, according to specs it should take
> 4 or 8GB DIMMs in pairs. Sounds like the server is split into multiple
> partitions.

I'm confused about that too.  There definitely seemed to be only 6GB of
available RAM, but there's no way I can see that memory might be
partitioned across different blades.  The blades look pretty independent
...

			regards, tom lane

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