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Re: Large (8M) cache vs. dual-core CPUs

From: Ron Peacetree <rjpeace(at)earthlink(dot)net>
To: mark(at)mark(dot)mielke(dot)cc, Scott Marlowe <smarlowe(at)g2switchworks(dot)com>
Cc: Bill Moran <wmoran(at)collaborativefusion(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Large (8M) cache vs. dual-core CPUs
Date: 2006-04-26 03:07:17
Message-ID: 8991088.1146020837420.JavaMail.root@elwamui-mouette.atl.sa.earthlink.net (view raw or flat)
Thread:
Lists: pgsql-performance
>Another benefit of Pentium D over AMD X2, at least until AMD chooses
>to switch, is that Pentium D supports DDR2, whereas AMD only supports
>DDR. There are a lot of technical pros and cons to each - with claims
>from AMD that DDR2 can be slower than DDR - but one claim that isn't
>often made, but that helped me make my choice:
>
>    1) DDR2 supports higher transfer speeds. I'm using DDR2 5400 on
>       the Intel. I think I'm at 3200 or so on the AMD X2.
>
>    2) DDR2 is cheaper. I purchased 1 Gbyte DDR2 5400 for $147 CDN.
>       1 Gbyte of DDR 3200 starts at around the same price, and
>       stretches into $200 - $300 CDN.
>
There's a logical fallacy here that needs to be noted.

THROUGHPUT is better with DDR2 if and only if there is enough data to be fetched in a serial fashion from memory.

LATENCY however is dependent on the base clock rate of the RAM involved.
So PC3200, 200MHz x2, is going to actually perform better than PC2-5400, 166MHz x4, for almost any memory access pattern except those that are highly sequential.

In fact, even PC2-6400, 200MHz x4, has a disadvantage compared to 200MHz x2 memory.
The minimum latency of the two types of memory in clock cycles is always going to be higher for the memory type that multiplies its base clock rate by the most.

For the mostly random memory access patterns that comprise many DB applications, the base latency of the RAM involved is going to matter more than the peak throughput AKA the bandwidth of that RAM.

The big message here is that despite engineering tricks and marketing claims, the base clock rate of the RAM you use matters.

A minor point to be noted in addition here is that most DB servers under load are limited by their physical IO subsystem, their HDs, and not the speed of their RAM.

All of the above comments about the relative performance of different RAM types become insignificant when performance is gated by the HD subsystem. 


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