RAM Speed vs. Ram Quantity
August 17, 2009 11:36 AM   Subscribe

Is there a general formula for determining the relative value of RAM speed versus RAM quantity? More specifically, am I better off with 2GB at DDR400 in dual-channel mode, or 2.5GB at DDR333 in single-channel mode?

I have an ASUS A8N5X with an Athlon 64 3200+ and have been running on a single 512 MB DIMM for way too long. I just bought 2GB (in the form of 2x1GB) and I've tested it both on its own and with the original 512 stick as well. By itself in the proper slots, the new ram runs at DDR 400 in dual-channel mode. With the old stick added in, the motherboard drops down into single-channel at 333.

The internet tells me this is correct behavior (specifically that the motherboard will drop to 333 MHz when running in single-channel mode), so the question is whether I'm better off with the extra 512 MB or with the faster speed.

Additional information that might be relevant: I still play games now and again (TF2 mostly, which ran largely fine on the old RAM), but the primary RAM-intensive use which finally prompted the upgrade is working with 10-megapixel RAW files while keeping a browser window and a half-dozen or so tabs open.

Right now I'm using the 2GB at the faster speed. Performance both ways seems to be fine (and a heck of a lot better than before), but I'm wondering if there's a good formula here. Also if it matters, I'm happy to play with CAS latency and timings and all that, but something tells me those effects will be comparatively small.
posted by Partial Law to Computers & Internet (9 answers total)
Depends on the application I guess. If your workflow works out in a way that your computer starts swapping out at right over 2gigs or RAM then you want more RAM, but that seems like an extreme case. Faster RAM is always better, if you dont usually exceed the amount.

I have an ASUS A8N5X with an Athlon 64 3200+

Thats, what, a 6 or 7 year old processor design now? I would think spending cash on a CPU upgrade to a dual core would give you better results than slightly faster RAM.
posted by damn dirty ape at 11:54 AM on August 17, 2009

It depends entirely on your workload and your operating system. In your specific case I'd stick wtih 2 gigs at 400MHz for pretty much any practical use.

If you want to understand this in detail for yourself, your first measure that can help you is page faults, either via the Resource Monitor in Windows or vmstat in Unix. Note that it's pretty hard to really understand this measure, particularly in a modern OS with mmap() and sophisticated memory management like Photoshop does.
posted by Nelson at 12:00 PM on August 17, 2009

There's no real formula for it. I don't remember the results from that era of processor, but these days the raw memory speed is responsible for a tiny fraction of the performance of the processor (that is, an i7 with a faster memory bus is only a few percent faster than with the stock memory speed).

I have an X2 3800+ on a A8N-SLI as a spare system, and I use 3 1GB DIMMs in it. There's probably a 5% performance hit for the loss of dual-channel, but I gain more than that when I use more than 2GB of RAM. It's not going to hurt much one way or another.
posted by Rendus at 12:29 PM on August 17, 2009

Forget the memory speed, it's not going to make a noticeable difference.

That said, as Rendus said above, it's the loss of dual-channel performance that you may notice.

You'll probably get more benefit with the extra 512 RAM, since you'll have an extra 512 megs to play with for your images and browsers. With the faster/dual-channel RAM you'd be hitting the swap file much quicker, negating the benefits of the extra speed.
posted by Sonic_Molson at 1:01 PM on August 17, 2009

What it comes down to is that dual channel memory can transfer data twice as fast as single channel memory, but single channel memory is something like 1000x faster at accessing data and ~100x as fast as transferring data than a hard disk. More memory typically means more data can be cached in RAM, which means less hard disk access. Avoidance of disk access is likely to be a much bigger win than doubling your peak RAM bandwidth. Besides, you were already using single channel memory before, so its not like you are giving up that advantage.

An A64 3200+ isn't a very fast CPU by todays standards, but its still pretty capable. The suggestion that you should have upgraded the CPU is just silly. The RAM upgrade is needed anyway (and you've already done it), and there are limited upgrade options for any motherboard with that chip. It's going to be a lot more to upgrade CPU + Motherboard + RAM than it is to just upgrade your RAM.
posted by Good Brain at 2:26 PM on August 17, 2009

Best answer: Single channel ddr333 has 166mhz * 64bits = 2.7Gb/sec bandwidth
Dual channel ddr333 has 166mhz * 64bits * 2 = 5.4Gb/sec bandwidth
Dual channel ddr400 has 200mhz * 64bits * 2 = 6.4Gb/sec bandwidth.

I would speculate your motherboard is functioning with 2.5GB at single channel ddr333 speeds, which provides ~ 42% of the bandwidth of 2GB at dual channel ddr400 speeds.

Whether or not this is noticed by the end user is questionable. Tomshardware found little difference in single vs dual channel in call of duty 2, prey and quake 4 while Tech Connect found significant differences in half life 2 but only at max graphics quality.

Memory timings have little impact upon performance unless you're overclocking, coverage from Tech Report.
posted by zentrification at 2:30 PM on August 17, 2009

Best answer: The flipside to "memory bandwidth isn't that important" is "more RAM is only useful if you use it". In my experience on a desktop system, even Vista x64, the working set for typical users is under 2 gigs. The rest of the RAM is useful as a filesystem cache, yes, but that has diminishing returns. The case where the extra RAM would really make a big difference is if you can actually use it for something. If your system is swapping to disk regularly, then absolutely you want more RAM.
posted by Nelson at 3:16 PM on August 17, 2009

The extra bandwidth would only matter if you were doing something that could saturate the memory throughput. I don't think there are a lot of things like that; maybe video compression. If you were doing something that did saturate the bus it would chew up memory pretty fast and you'd still be better off with the extra 512M since it would put off paging to disk for longer.
posted by chairface at 8:23 PM on August 17, 2009

Best answer: Disk cache has diminishing returns, but given the huge differences between RAM and disk speed, even relatively small improvements in cache hit rate can still have a big impact on performance. I suspect that on a 2GB machine, an extra 512MB could add 50-100% to the ram available for disk cache.

I'd keep an eye on the size of available memory and the system cache size in Task Manager after using the machine for a while and the same set of applications open, with and without the 512 installed. If the available memory is typically near or over 512MB with the extra memory installed then it is clearly not doing you much good. Similarly, if available memory is really low without the 512 installed, you'd likely benefit from it. In between those extremes, I'm not sure.
posted by Good Brain at 12:56 PM on August 18, 2009

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