Well, DDR chips - quality ones,
at least - consume a little less power than would SDR at the same speed
and voltage (not that any SDR does run at the same speed - but there's nothing
stopping you from extrapolating the graphs and pretending). So you're talking
maybe 0.6 watts per chip, and less than ten watts per 16 chip 256Mb module,
at 3.3 volts.
Except PC DDR memory modules
don't run at 3.3 volts. They run at 2.5 volts. So they actually draw less
power.
Micron have, by the way, obligingly
created a couple of nice up-to-date System Power Calculator spreadsheets
in Microsoft Excel format, which you can download from here
if you want to figure out the power consumption and resultant heat output
of your memory modules. You'll have to browse some data sheets to work out
how to use the calculators properly, though.
One kind of PC system memory
does come with heat sinks attached. RDRAM. It does it for a reason, but
it's not necessarily the reason you think.
The metal plates on RDRAM modules
- which look very like the ones in the Thermaltake kit - aren't very good
heat sinks. They're heat spreaders.
Stick any old chunk of metal
to the top of your RAM with something reasonably thermally conductive and
it'll help the memory stay cooler. You really need lots of surface area
and/or lots of fan-assisted air flow to make a big difference, though; these
plates don't provide that. Which is OK, because RDRAM doesn't need it.
RDRAM modules do need heat spreaders,
though.
SDRAM - SDR or DDR - is basically
parallel in design. Each chip on a RAM module contributes 8 or 16 bits (depending
on the module type) to each 64 bit wide burst data transfer.
RDRAM, in comparison, is basically
serial. It can have eight or 16 chips supplying one 16 bit wide channel
with data (at a much higher clock speed than SDRAM, which makes up for the
narrow channels), but only one chip out of that 8 or 16 will be used for
each burst transfer. That chip will perform four operations to make up a
64 bit block which, in an SDRAM setup, would be delivered in one operation
by four, or eight, chips. When an RDRAM chip's performing an operation it's
running at pretty much full power, and emitting pretty much full heat.
If one RDRAM chip did one 64
bit burst and then another did the next, then the heat distribution would
still be very even. Each burst takes very little time, and causes very little
heat production, by itself. But, actually, just one chip on an RDRAM module
is commonly called upon to do burst after burst after burst. The total power
consumption can thus be concentrated onto one chip at a time.
Hence, the heat spreaders. They
let the chips share the heat, even when they're not sharing the load. Getting
the heat into the air is the spreaders' secondary function.
When your memory already shares
its heat production, as SDRAM does, then heat spreaders are... well, they're
not the most useful thing in the world. Let's put it that way.
Theorising is all very well;
how about some empirical tests, eh?
To find out just how hot SDRAM
might get, I stuck a thermal probe to one of the chips on the OCZ module
- on the side away from the processor cooler breeze - while running it at
133MHz with full tweaks applied. Then, I started the handy intensive memory
test program DocMemory
flogging away in "burn in" mode, where it just runs through all of its tests
over and over.
The temperature difference between
the probe on the RAM and another one sitting in the air away from the case
was only eight degrees Centigrade.
This was in a case with the
side removed, which is the quick and dirty way to get ultra-maximum air
flow. Nonetheless, the area around the CPU, including the RAM slots, was
significantly warmer than ambient, thanks to the presence of the steaming
air-cooled Athlon. When I moved my ambient-temperature probe so that it
was sampling the air next to the video card, on the side towards the CPU,
the temperature therefore rose. By seven degrees Centigrade.
Which made the RAM temperature
a big one degree C above the local ambient.
Now, my dual-input thermometer
is not the world's most accurate, and my stuck-on thermal probe wasn't giving
me exactly the temperature of the RAM. It could easily have been a few degrees
off. So let's say the real delta-T, the difference in temperature between
the surface of the RAM chip and the air inside the case, was actually five
degrees Centigrade.
Assuming a third of a watt of
heat output from the chip the probe was attached to, that'd work out at
a thermal resistance figure of 15 degrees Centigrade per watt. Which is
lousy, and pretty much what you'd expect from a chip with a blob of thermal-probe-retention
stickum on it. The other chips, without probe-sticking goop insulating them,
no doubt cooled better.
But with only a third of a watt
of heat output - and probably a lot less, even during intensive computing
tasks, as discussed above - it doesn't matter if you've got lousy cooling.
You could encase the whole darn RAM module in a silicone mummy-wrapping
and it still probably wouldn't overheat.
DDR memory emits even less heat,
at least if comparisons like this
one are to be believed. So heat sinking will be even less
useful for DDR than for SDR. It's tinsel, pure and simple.
So I really cannot see what,
in the name of all that is small, furry and adorable, is the point of putting
cooling thingies on an SDRAM module. There's bog-all heat there in the first
place.
The fact that Thermaltake sells
DDR RAM with heat spreaders pre-installed doesn't mean much. Not that the
argument from authority is one of my favourites, or anything, but if Micron
started doing it, that'd interest me.
On their
page for this particular RAM accessory, Thermaltake explain
"High Performance Heat Spreader makes the SDRAM running well as overclocking
the system!!!"
I think there's something in
that for all of us, don't you?
They also include some low-legibility
temperature graphs (which aren't any more legible in the Adobe Acrobat spec
sheet here)
which seem to suggest that all of the memory chips are at the same temperature
all over. The graphs also say that some unspecified DDR RAM module running
at some unspecified speed with an unknown ambient temperature and a highly
suspiciously straight temperature gain graph only actually ran about five
degrees cooler - about 40 versus about 45 degrees Centigrade - when the
heat spreader was installed.
Assuming the roughly 7°C starting
temperature to be the ambient - for no tremendously good reason - and assuming
other system components had nothing to do with the result - which is pretty
much completely contrary to reason - and assuming that the utterly undescribed
experimental parameters weren't prejudicial to the result - I
thank God I wore my corset, because I think my sides have split
- then the heat spreaders made a bit more than 10% difference, versus no
special cooling at all.
Accordingly, allow me to suggest
an equally useful RAM-cooling technology.
