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Re: hyperthreaded cpu still an issue in 8.4?

From: Matthew Wakeling <matthew(at)flymine(dot)org>
To: Dave Youatt <dave(at)meteorsolutions(dot)com>
Cc: Greg Smith <gsmith(at)gregsmith(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: hyperthreaded cpu still an issue in 8.4?
Date: 2009-07-29 11:16:41
Message-ID: alpine.DEB.2.00.0907291210210.19493@aragorn.flymine.org (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, 28 Jul 2009, Dave Youatt wrote:
> Unlikely. Different threads on the same CPU core share their resources, so they don't
> need an explicit communication channel at all (I'm simplifying massively here). A real
> interconnect is only needed between CPUs and between different cores on a CPU, and of
> course to the outside world. Scott's explanation of why SMT works better now is much more
> likely to be the real reason.

Actually, no, I wrote that. Please give at least some indication when 
replying to an email which parts of it are your words and which are quotes 
from someone else. Emails can be incredibly confusing without that 
distinction.

You actually wrote:

> :-) there's also this interconnect thingie between sockets, cores and memory. Nehalem has
> a new one (for Intel), integrated memory controller, that is.  And a new on-chip cache
> organization.

This, (like I mention elsewhere) will make the CPU faster overall, but is 
unlikely to increase the performance gain of switching SMT on. In fact, 
having a lower latency memory controller is more likely to reduce some of 
the problem that SMT is trying to address - that of a single thread 
stalling on memory access.

Having said that, memory access latency is not scaling as quickly as CPU 
speed, so over time SMT is going to get more important.

Matthew

-- 
"Take care that thou useth the proper method when thou taketh the measure of
 high-voltage circuits so that thou doth not incinerate both thee and the
 meter; for verily, though thou has no account number and can be easily
 replaced, the meter doth have one, and as a consequence, bringeth much woe
 upon the Supply Department."   -- The Ten Commandments of Electronics

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Subject: Re: hyperthreaded cpu still an issue in 8.4?
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