About ideal RAM capacity

[O. Hartmann]

Andrew Pantyukhin wrote:
> On 2/17/07, satimis <satimis at yahoo.com> wrote:
>>
>> Hi folks,
>>
>> I'm prepared to run FreeBSD-6.2 amd64 on an AMD Athlon64 x2 (dualcore)
>> 512Kx2 3800 box as server.  What will be an ideal RAM capacity to be
>> installed.  TIA
>
> You can perform a binary search, benchmarking 2^n Mb of RAM
> installed where n is 10..infinity. Athlon64 x2 motherboards
> with support for over 64 Gb are hard to come by nowadays,
> though.
>
> Good luck!
At this moment, you will be limited to 4GB RAM for single socket
Opterons (1xxx series for AM2, 1xx series for socket 939). The 4GB
limitation is a hard limit for socket 939 boxes, due to the memory
controler is incapable of handling more than 4 memory slots divided into
two channels and due to JEDEC limitations for DDR400 RAM each memory
stick can hold up to 1GB.
For AM2 socket single Opteron boards or Athlon64 chips the limitation is
due to the lack of suitable and fast (and affordable) 2GB sticks
(DDR2-800). Multisocket Opterons use a different memory controller which
is capable of handling registered memory sticks, so the capacity of each
stick can rise up to 2 GB or 4 GB per stick.
Unfortunately, AMD did not offer their single socket Opterons (neither
Socket 939, nor AM2) with a registered memory controller, so they simply
are the same as their Athlon64 counterparts of the same production line,
but you pay sometimes as double as much for the brand 'Opteron'. Nearby,
it would have been a great benefit if someone could apply Athlon64 chips
to a server motherboard (like TYAN's Socket 939 or AM2 Opteron server
boards) for low cost/entry purposes with memory limit to 4GB and then
upgrade to Opteron/single socket with an integrated registered memory
controller to raise the memory limit up to 8 or 16 GB (socket 939 or AM2
respective). But I do not know whether we need more pins wiring from the
socket to the memory slots in case of registered memory support (I doubt
that).

So, sorry for the long explanations and bothering, your memory demand
depends on what your server will 'serve'. In many cases, 2GB memory is
enough, but for database servers, webservers with an expected high load
4GB RAM would be eligible.

If your Athlon64 box is an outdated Socket 939, you should be aware of
the fact, that the memory controller slows down its speed from DDR400 to
DDR333 and command-T cycle from 1T to 2T if you apply 4GB! In most cases
you still are able to run them stable with 200/400 MHz (high quality
memory), but command cycle latency is then set to 2T. If memory speed
isn't the point, forget about what I said.

Regards,
Oliver