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

From: Scott Marlowe <smarlowe(at)g2switchworks(dot)com>
To: Bill Moran <wmoran(at)collaborativefusion(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Large (8M) cache vs. dual-core CPUs
Date: 2006-04-25 18:33:38
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Lists: pgsql-performance
On Tue, 2006-04-25 at 13:14, Bill Moran wrote:
> I've been given the task of making some hardware recommendations for
> the next round of server purchases.  The machines to be purchased
> will be running FreeBSD & PostgreSQL.
> Where I'm stuck is in deciding whether we want to go with dual-core
> pentiums with 2M cache, or with HT pentiums with 8M cache.

Given a choice between those two processors, I'd choose the AMD 64 x 2
CPU.  It's a significantly better processor than either of the Intel
choices.  And if you get the HT processor, you might as well turn of HT
on a PostgreSQL machine.  I've yet to see it make postgresql run faster,
but I've certainly seen HT make it run slower.

If you can't run AMD in your shop due to bigotry (let's call a spade a
spade) then I'd recommend the real dual core CPU with 2M cache.  Most of
what makes a database slow is memory and disk bandwidth.  Few datasets
are gonna fit in that 8M cache, and when they do, they'll get flushed
right out by the next request anyway.

> Does anyone in the PostgreSQL community have any experience with
> large caches or dual-core pentiums that could make any recommendations?
> Our current Dell 2850 systems are CPU bound - i.e. they have enough
> RAM, and fast enough disks that the CPUs seem to be the limiting
> factor.  As a result, this decision on what kind of CPUs to get in
> the next round of servers is pretty important.

If the CPUs are running at 100% then you're likely not memory I/O bound,
but processing speed bound.  The dual core will definitely be the better
option in that case.  I take it you work at a "Dell Only" place, hence
no AMD for you...

Sad, cause the AMD is, on a price / performance scale, twice the
processor for the same money as the Intel.

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