My deskside computer has 4 DIMM slots. I don't know any
more how this stuff is wired, so my question:

Will two 8 GB DIMMs perform better than a single 16 GB DIMM?

"jason_warren@ieee.org" <jason_warren@ieee.org> wrote:

> My deskside computer has 4 DIMM slots. I don't know any
> more how this stuff is wired,

Neither does anyone, because you didn't mention brand and model of your
motherboard. With that info, the mobo's manual probably says if dual
channel mode is support by matching DIMMs, and in which slots.

> so my question:
>
> Will two 8 GB DIMMs perform better than a single 16 GB DIMM?

Perhaps, might depend on which mobo slots are used for the DIMMs.

In article <x0bunokfytic.dlg@v.nguard.lh>, V@nguard.LH says...
>
> "jason_warren@ieee.org" <jason_warren@ieee.org> wrote:
>
> > My deskside computer has 4 DIMM slots. I don't know any
> > more how this stuff is wired,
>
> Neither does anyone, because you didn't mention brand and model of your
> motherboard. With that info, the mobo's manual probably says if dual
> channel mode is support by matching DIMMs, and in which slots.
>
> > so my question:
> >
> > Will two 8 GB DIMMs perform better than a single 16 GB DIMM?
>
> Perhaps, might depend on which mobo slots are used for the DIMMs.

It's an Alienware R9 (AMD) if that's helpful. There's nothing in
documentation about this that I can find,

"jason_warren@ieee.org" <jason_warren@ieee.org> wrote:

> In article <x0bunokfytic.dlg@v.nguard.lh>, V@nguard.LH says...
>>
>> "jason_warren@ieee.org" <jason_warren@ieee.org> wrote:
>>
>>> My deskside computer has 4 DIMM slots. I don't know any
>>> more how this stuff is wired,
>>
>> Neither does anyone, because you didn't mention brand and model of your
>> motherboard. With that info, the mobo's manual probably says if dual
>> channel mode is support by matching DIMMs, and in which slots.
>>
>>> so my question:
>>>
>>> Will two 8 GB DIMMs perform better than a single 16 GB DIMM?
>>
>> Perhaps, might depend on which mobo slots are used for the DIMMs.
>
> It's an Alienware R9 (AMD) if that's helpful. There's nothing in
> documentation about this that I can find,

That's a Dell computer, and Dell is horrendous at providing technical
details in their manuals.

https://www.dell.com/support/manuals/en-us/alienware-aurora-r9-desktop/alienware-aurora-r9-setup-and-specifications/

Eventually I found:

https://www.dell.com/support/home/en-us/product-support/product/alienware-aurora-r9-desktop/drivers

Picked Documentation -> Manuals and Documents. Dell sells by specs, so
even when you order the same model you could end up with different parts
than another build for the same model. The manual only tells you there
are 4 memory slots which can use 8GB or 16GB modules and a table of max
memory configurations.

In the service manual, table 2, shows a memory config matrix. They show
using the XMM1 and XMM2 slots when installing 2 memory modules. If you
go back to page 10 showing the components on the mobo, those slots are
near each other, but in different slot groups. Looks like they want the
2 modules to be 1 in each slot group, so dual-channel gets used
(different channels for each slot group). However, they don't say why
you should use XMM1 and XMM2 slots, but that's the config they suggest.

You might see a difference (improvement) in memory performance with two
8GB modules on different channels (XMM1 & XMM2) when using benchmarks,
but you'll unlikely notice any significant performance boost when using
the computer.

What size memory module(s) do you know have in the mobo memory slots?
If you have one 8 GB module, add another 8 GB module. Are you thinking
of replacing the current 8GB module with one 16GB module, or by keeping
the old one and adding another 8GB module? If so, are you running out
of memory? Remember that unused memory is wasted memory, but if you can
afford it then put in the max memory the mobo will support.

On 12/17/2023 2:09 AM, jason_warren@ieee.org wrote:
> In article <x0bunokfytic.dlg@v.nguard.lh>, V@nguard.LH says...
>>
>> "jason_warren@ieee.org" <jason_warren@ieee.org> wrote:
>>
>>> My deskside computer has 4 DIMM slots. I don't know any
>>> more how this stuff is wired,
>>
>> Neither does anyone, because you didn't mention brand and model of your
>> motherboard. With that info, the mobo's manual probably says if dual
>> channel mode is support by matching DIMMs, and in which slots.
>>
>>> so my question:
>>>
>>> Will two 8 GB DIMMs perform better than a single 16 GB DIMM?
>>
>> Perhaps, might depend on which mobo slots are used for the DIMMs.
>
> It's an Alienware R9 (AMD) if that's helpful. There's nothing in
> documentation about this that I can find,
>

Your first pair, go in the ones with the white tabs.

https://i.ebayimg.com/images/g/Z1oAAOSwgCdhcHVh/s-l1600.jpg

It uses DDR4.

https://www.dell.com/support/manuals/en-ca/alienware-aurora-r9-desktop/alienware-aurora-r9-setup-and-specifications/memory?guid=guid-b0024580-18d7-4e8e-b494-84193888b9ee&lang=en-us

Configurations supported:

8 GB DDR4 at 2666 MHz
16 GB DDR4 at 2666 MHz
32 GB DDR4 at 2666 MHz
64 GB DDR4 at 2666 MHz

16 GB XMP at 2933 MHz (DDR4 memory, with XMP table entries in the SPD EEPROM)
32 GB XMP at 2933 MHz
64 GB XMP at 2933 MHz

16 GB XMP at 3200 MHz (DDR4 memory, with XMP table entries in the SPD EEPROM)
32 GB XMP at 3200 MHz
64 GB XMP at 3200 MHz

XMP is a BIOS switch. And if you have a matched pair of XMP DIMMs,
you can switch on XMP. You would buy a pair with XMP specs to
match motherboard capability.

Enter the BIOS and verify there is an XMP switch on one of the BIOS pages.
At least, if you expect to use XMP configured DIMMs.

XMP DIMMs don't have to run in XMP mode, but the other choices
in their table, are pretty lame.

Generally speaking, Profile Number 1, is CR2 (Command Rate 2). The
command bus has a value placed on it for two cycles, it is strobed in
on the second cycle. This is the "conservative" profile. This gives
extra setup time on address/command. It's not a data bus issue.
Strangely, the BIOS does not label the options, in terms of their
aggression or whether the profile is CR2 or CR1.

The Profile Number 2, is CR1 (Command Rate 1). That's a more
aggressive setting. It might give a Streams benchmark that is
5% better or so. The DIMMs also get a tiny bit warmer :-)
Since you have air space between DIMMs when only two are used,
there's no thermal concerns. This was measured, when by accident,
I used Command Rate 1 by selecting the wrong profile.

Profile 1 38.4GB/sec <=== Machine runs this way now.
Profile 2 (aggressive) 40.8GB/sec <=== My first memtest was too high

7ZIP is the only utility that benefits from all that fuss.
For most other activities, even the 2666 choice will work
with no perceptible impact. Doubling the price of the DIMMs,
really buys you nothing. Expensive DIMMs are just for benching.

DIMM # 1
SMBus address 0x50
Memory type DDR4
Module format UDIMM
Module Manufacturer(ID) G.Skill
SDRAM Manufacturer (ID) SK Hynix
Size A lot
Max bandwidth DDR4-3200 (1600 MHz)
Max JEDEC DDR4-2666 (1333 MHz)
Part number F4-3200C16-32GVK
Nominal Voltage 1.20 Volts
EPP no
XMP yes, rev. 2.0
AMP no <=== AMD system does show XMP profile
JEDEC timings table CL-tRCD-tRP-tRAS-tRC @ frequency
JEDEC #1 10.0-11-11-24-34 @ 733 MHz
JEDEC #2 11.0-11-11-26-37 @ 800 MHz
JEDEC #3 12.0-12-12-28-40 @ 866 MHz
JEDEC #4 13.0-13-13-30-43 @ 933 MHz
JEDEC #5 14.0-15-15-34-48 @ 1033 MHz
JEDEC #6 15.0-16-16-36-51 @ 1100 MHz
JEDEC #7 16.0-17-17-38-54 @ 1166 MHz
JEDEC #8 17.0-17-17-40-57 @ 1233 MHz
JEDEC #9 18.0-18-18-42-60 @ 1300 MHz
JEDEC #10 19.0-19-19-43-61 @ 1333 MHz
JEDEC #11 20.0-19-19-43-61 @ 1333 MHz <=== conventional speed choice (before you switch on XMP)
XMP profile XMP-3200
Specification DDR4-3200
VDD Voltage 1.350 Volts <=== Enthusiast RAM gets enthusiast voltage (XMP switch)
Min Cycle time 0.625 ns (1600 MHz)
Max CL 16.0
Min tRP 11.25 ns
Min tRCD 11.25 ns
Min tRAS 23.75 ns
Min tRC 35.00 ns
Min tRRD 2.50 ns
XMP timings table CL-tRCD-tRP-tRAS-tRC-CR @ frequency (voltage)
XMP #1 16.0-18-18-38-56-n.a @ 1600 MHz (1.350 Volts) <=== BIOS sees two profiles so
/ the information seems incomplete.
DDR4-3200 -----------/ The CR is "n.a" for some reason.
Paul

In article <MPG.3fe868c9eae30f34989697@reader80.eternal-september.org>,
jason_warren@ieee.org says...
>
> In article <x0bunokfytic.dlg@v.nguard.lh>, V@nguard.LH says...
> >
> > "jason_warren@ieee.org" <jason_warren@ieee.org> wrote:
> >
> > > My deskside computer has 4 DIMM slots. I don't know any
> > > more how this stuff is wired,
> >
> > Neither does anyone, because you didn't mention brand and model of your
> > motherboard. With that info, the mobo's manual probably says if dual
> > channel mode is support by matching DIMMs, and in which slots.
> >
> > > so my question:
> > >
> > > Will two 8 GB DIMMs perform better than a single 16 GB DIMM?
> >
> > Perhaps, might depend on which mobo slots are used for the DIMMs.
>
> It's an Alienware R9 (AMD) if that's helpful. There's nothing in
> documentation about this that I can find,

V@ngaurd & Paul:

"Your first pair, go in the ones with the white tabs."

Uhoh... The service manual picture of the mobo shows sockets
with black/white-clip sockets alternating: b/w/b/w.
But the mobo in my machine has two white clips in the
middle two positions, NOT alternating as in the picture.
So, I'm not sure how to proceed. I may just buy 3 more
8GB DIMMs and populate all four slots :)

"jason_warren@ieee.org" <jason_warren@ieee.org> wrote:

> In article <MPG.3fe868c9eae30f34989697@reader80.eternal-september.org>,
> jason_warren@ieee.org says...
>>
>> In article <x0bunokfytic.dlg@v.nguard.lh>, V@nguard.LH says...
>>>
>>> "jason_warren@ieee.org" <jason_warren@ieee.org> wrote:
>>>
>>> > My deskside computer has 4 DIMM slots. I don't know any
>>> > more how this stuff is wired,
>>>
>>> Neither does anyone, because you didn't mention brand and model of your
>>> motherboard. With that info, the mobo's manual probably says if dual
>>> channel mode is support by matching DIMMs, and in which slots.
>>>
>>> > so my question:
>>> >
>>> > Will two 8 GB DIMMs perform better than a single 16 GB DIMM?
>>>
>>> Perhaps, might depend on which mobo slots are used for the DIMMs.
>>
>> It's an Alienware R9 (AMD) if that's helpful. There's nothing in
>> documentation about this that I can find,
>
> V@ngaurd & Paul:
>
> "Your first pair, go in the ones with the white tabs."
>
> Uhoh... The service manual picture of the mobo shows sockets
> with black/white-clip sockets alternating: b/w/b/w.
> But the mobo in my machine has two white clips in the
> middle two positions, NOT alternating as in the picture.
> So, I'm not sure how to proceed. I may just buy 3 more
> 8GB DIMMs and populate all four slots :)

As I cited from the service manual, you first use XMM1 and XMM2 slots
together first, then secondly use XMM3 and XMM4 slots together. There
should be silkscreening on the mobo to show the names of the slots. The
manual has a photo showing the mobo and the slot names.

In the photo, #26 and #27 are the XMM1 (white tab) and XMM2 (black)
slots.

Be careful if you get memory modules with heat sinks. Some are so fat
that you cannot have them in adjacent mobo slots. In the photo, XMM1
and XMM2 are a little bit farther away from each other than XMM3 to XMM1
and XMM2 to XMM4. Unless you are overclocking (didn't bother to check
if this mobo's BIOS has any overclock settings, especially for
pre-builts where BIOS is often minimalized), don't bother with heatsinks
on the memory modules. Just be sure you haven't blocked air flow inside
the case with wide cabling or other obstructions. From what I see in
the service manual's pic of the whole case, the power supply swings out
to gain access to the mobo, so the heatsinks on the memory modules being
too high could also cause problems when swinging the PSU back down.
Just use bare memory modules. Like with laptops, this condensed tower
is cramped inside. You might have to find low-profile memory modules.

On 12/17/2023 5:52 PM, jason_warren@ieee.org wrote:
> In article <MPG.3fe868c9eae30f34989697@reader80.eternal-september.org>,
> jason_warren@ieee.org says...
>>
>> In article <x0bunokfytic.dlg@v.nguard.lh>, V@nguard.LH says...
>>>
>>> "jason_warren@ieee.org" <jason_warren@ieee.org> wrote:
>>>
>>>> My deskside computer has 4 DIMM slots. I don't know any
>>>> more how this stuff is wired,
>>>
>>> Neither does anyone, because you didn't mention brand and model of your
>>> motherboard. With that info, the mobo's manual probably says if dual
>>> channel mode is support by matching DIMMs, and in which slots.
>>>
>>>> so my question:
>>>>
>>>> Will two 8 GB DIMMs perform better than a single 16 GB DIMM?
>>>
>>> Perhaps, might depend on which mobo slots are used for the DIMMs.
>>
>> It's an Alienware R9 (AMD) if that's helpful. There's nothing in
>> documentation about this that I can find,
>
> V@ngaurd & Paul:
>
> "Your first pair, go in the ones with the white tabs."
>
> Uhoh... The service manual picture of the mobo shows sockets
> with black/white-clip sockets alternating: b/w/b/w.
> But the mobo in my machine has two white clips in the
> middle two positions, NOT alternating as in the picture.
> So, I'm not sure how to proceed. I may just buy 3 more
> 8GB DIMMs and populate all four slots :)
>

The position in this picture, is the position you want.
The things that are white (let's pretend they are called
slot #2 and slot #4) are the ones. I don't know how your
user manual labels them, for enumeration.

https://i.ebayimg.com/images/g/Z1oAAOSwgCdhcHVh/s-l1600.jpg

This is the bus arrangement.
|| || || ||
Channel0 || || || ||
CPU memory controller ---------- Slot1 --- Slot2 || ||
|| || || ||
Channel1 || || || ||
---------------------------- Slot3 --- Slot4
|| || || ||
|| || || ||
DIMM DIMM
here here

You will be placing one DIMM on each channel. The DIMM goes on
the end of its channel.

*******

Unlike early computers, which had certain restrictions on population,
modern computers will accept any memory configuration. Yours is a modern one.

1GB 2GB --- 8GB

However, if you chose to do it "a strange way", you will not get the
best performance. As shown in that example, the computer will definitely
register 11GB. But the transfer speed may vary with address space (I've
actually measured this, with a modified copy of memtest, on an NForce2).
(Physical to virtual is mapped straight-thru in a memtest.)

The absolute best performance, comes from four double-sided DIMMs.

16GB 16GB 16GB 16GB

The reason for that, is the number of "open pages" is maximized if
four double-sided DIMMs are used. A DDR4 is likely to be double-sided
at 16GB. The 8GB module might be single sided. (No, the pictures of
the product hide this detail. You can hardly ever tell visually, what
you're getting.) The pricing on the memory chips, determines whether
they come single-sided or double-sided.

However, the improvement from interleaving over eight banks of
open pages, is around 2%, and nobody gives a rats ass about
stuff like this. Your configuration will give most of the performance
improvement to be expected via dual-channel operation.

--- 8GB --- 8GB

Increasing the operating frequency of DIMMs, is deceptive. Once you
get above the memory controller maximum frequency, it uses a "gear changer"
and cuts one of the speeds in half. Then, to make up for that, the
DIMM must be pushed much higher in frequency. There is also a divide by 4
setting, which ruins things if you go even higher. To tune in that case
then, is a lot of work fiddling with stuff. And again, small percentage
improvements result.

When you see DIMMs that are twice the price of other DIMMs, the "utility"
of this now, is limited. Enthusiasts building "race cars", would likely
use two small single-sided DIMMs, for max clock frequency. And toast the
living shit out of the DIMM. Maybe their DDR4 would be at 5000, their
divider at 2.

Paul

Paul <nospam@needed.invalid> wrote:

> On 12/17/2023 5:52 PM, jason_warren@ieee.org wrote:
>> In article <MPG.3fe868c9eae30f34989697@reader80.eternal-september.org>,
>> jason_warren@ieee.org says...
>>>
>>> In article <x0bunokfytic.dlg@v.nguard.lh>, V@nguard.LH says...
>>>>
>>>> "jason_warren@ieee.org" <jason_warren@ieee.org> wrote:
>>>>
>>>>> My deskside computer has 4 DIMM slots. I don't know any
>>>>> more how this stuff is wired,
>>>>
>>>> Neither does anyone, because you didn't mention brand and model of your
>>>> motherboard. With that info, the mobo's manual probably says if dual
>>>> channel mode is support by matching DIMMs, and in which slots.
>>>>
>>>>> so my question:
>>>>>
>>>>> Will two 8 GB DIMMs perform better than a single 16 GB DIMM?
>>>>
>>>> Perhaps, might depend on which mobo slots are used for the DIMMs.
>>>
>>> It's an Alienware R9 (AMD) if that's helpful. There's nothing in
>>> documentation about this that I can find,
>>
>> V@ngaurd & Paul:
>>
>> "Your first pair, go in the ones with the white tabs."
>>
>> Uhoh... The service manual picture of the mobo shows sockets
>> with black/white-clip sockets alternating: b/w/b/w.
>> But the mobo in my machine has two white clips in the
>> middle two positions, NOT alternating as in the picture.
>> So, I'm not sure how to proceed. I may just buy 3 more
>> 8GB DIMMs and populate all four slots :)
>>
>
> The position in this picture, is the position you want.
> The things that are white (let's pretend they are called
> slot #2 and slot #4) are the ones. I don't know how your
> user manual labels them, for enumeration.
>
> https://i.ebayimg.com/images/g/Z1oAAOSwgCdhcHVh/s-l1600.jpg
>
> This is the bus arrangement.
> || || || ||
> Channel0 || || || ||
> CPU memory controller ---------- Slot1 --- Slot2 || ||
> || || || ||
> Channel1 || || || ||
> ---------------------------- Slot3 --- Slot4
> || || || ||
> || || || ||
> DIMM DIMM
> here here
>
> You will be placing one DIMM on each channel. The DIMM goes on
> the end of its channel.
>
> *******
>
> Unlike early computers, which had certain restrictions on population,
> modern computers will accept any memory configuration. Yours is a modern one.
>
> 1GB 2GB --- 8GB
>
> However, if you chose to do it "a strange way", you will not get the
> best performance. As shown in that example, the computer will definitely
> register 11GB. But the transfer speed may vary with address space (I've
> actually measured this, with a modified copy of memtest, on an NForce2).
> (Physical to virtual is mapped straight-thru in a memtest.)
>
> The absolute best performance, comes from four double-sided DIMMs.
>
> 16GB 16GB 16GB 16GB
>
> The reason for that, is the number of "open pages" is maximized if
> four double-sided DIMMs are used. A DDR4 is likely to be double-sided
> at 16GB. The 8GB module might be single sided. (No, the pictures of
> the product hide this detail. You can hardly ever tell visually, what
> you're getting.) The pricing on the memory chips, determines whether
> they come single-sided or double-sided.
>
> However, the improvement from interleaving over eight banks of
> open pages, is around 2%, and nobody gives a rats ass about
> stuff like this. Your configuration will give most of the performance
> improvement to be expected via dual-channel operation.
>
> --- 8GB --- 8GB
>
> Increasing the operating frequency of DIMMs, is deceptive. Once you
> get above the memory controller maximum frequency, it uses a "gear changer"
> and cuts one of the speeds in half. Then, to make up for that, the
> DIMM must be pushed much higher in frequency. There is also a divide by 4
> setting, which ruins things if you go even higher. To tune in that case
> then, is a lot of work fiddling with stuff. And again, small percentage
> improvements result.
>
> When you see DIMMs that are twice the price of other DIMMs, the "utility"
> of this now, is limited. Enthusiasts building "race cars", would likely
> use two small single-sided DIMMs, for max clock frequency. And toast the
> living shit out of the DIMM. Maybe their DDR4 would be at 5000, their
> divider at 2.
>
> Paul

Note that you and I are giving the OP differing suggestions for memory
slot configuration. I used the service manual for his computer. What
are you basing your advice?

On 12/18/2023 12:14 AM, VanguardLH wrote:

>
> Note that you and I are giving the OP differing suggestions for memory
> slot configuration. I used the service manual for his computer. What
> are you basing your advice?
>

Mainly electrical engineering. Here is my parchment :-)

+------+
| |
| |
+------+

Our instructions actually agree.

Here is why they agree. They did a weirdness on slot numbering, marked in the picture.

[Picture]

https://i.postimg.cc/SQJw3yRY/Aurora-RAM-Table-Check-Their-Slot-Numbers.gif

They're actually filling from the bus ends,
once you undo their choice of slot numbering
schemes. Which means, you and I agree on
fill order. I'm filling from the end too.
Look at where their 1 and 2 are.

Paul

In article <ulpdjj$3g7p9$1@dont-email.me>, nospam@needed.invalid says...
>
> On 12/18/2023 12:14 AM, VanguardLH wrote:
>
> >
> > Note that you and I are giving the OP differing suggestions for memory
> > slot configuration. I used the service manual for his computer. What
> > are you basing your advice?
> >
>
> Mainly electrical engineering. Here is my parchment :-)
>
> +------+
> | |
> | |
> +------+
>
> Our instructions actually agree.
>
> Here is why they agree. They did a weirdness on slot numbering, marked in the picture.
>
> [Picture]
>
> https://i.postimg.cc/SQJw3yRY/Aurora-RAM-Table-Check-Their-Slot-Numbers.gif
>
> They're actually filling from the bus ends,
> once you undo their choice of slot numbering
> schemes. Which means, you and I agree on
> fill order. I'm filling from the end too.
> Look at where their 1 and 2 are.
>
> Paul

I've noticed other discrepancies between what's shown in
the service manual and what I actually see inside the machine.
I think I'll solve the problem by populating each socket with
an 8GB DIMM... eventually.

Thank you both.

Jason