HIGHMEM which Split do I want?

[xvalentinex]

Hey guys,

My apologies if this has been asked dozens of times. I would have assumed it has, but couldn't find anything through searches, perhaps my search skills suck.

I have a PC I use as a desktop. It has 4 512MB sticks of ram for 2GB total. I noticed recently that free and top only show 880MB of ram. Searching the google, gentoo wiki, and such, it is because I didn't enable HIGHMEM 4GB support. That's all well and good, and I am about to re-compile the kernel for that. However, I'm just curious what Memory Split some of you out there are using? I'm going to try 2G/2G User/Kernel, just because that sounds safe.

Anyone care to explain the differences between the Memory Splits without going to low level for me? And, which ones will give me the best performance in a desktop environment?

Thanks,
-Shem

[jlh]

I'm no expert, but I really think you want the normal split, which is 1GB for kernel and 3GB for user space. There's no need to change that unless you have a real reason to do it otherwise (and know what you're doing).

As of my understanding, there isn't any performance difference. It's just a difference in how memory is mapped into a process' space. I guess that means that if you use the 2/2 split, one single process can never use more than 2GB at once. With the 1/3 split it will be 3GB. (Which doesn't really matter if you only have 2GB to begin with.) But there's no reason to give more than 1GB to kernel-space unless you know you need more.

Also there are implications with binary modules (probably like non-open-source video drivers) if you use something else than the default. See the kernel's help entry on that.

[kfiaciarka]

If you enable "Compile kernel for small systems" you will have option split for 1gb lowmem

[bubbl07]

This article and the subsequent one are good reference points for this, even though they primarily discuss Windows. It goes into some detail, but I'll try to summarize what you need to know to the best of my ability.

Because of the 4 GB memory barrier inherent in 32-bit operating systems (you'll notice that the split option doesn't exist in the kernel configuration for linux x86_64), you'll have to decide how to allocate memory to share between the kernel (like driver modules) and userspace programs (like X). When I was using 32-bit gentoo on my old P4, I had it set to the default 1G kernel / 3G userspace, but it didn't really matter because I only had 512 MB of RAM in that box. Since you only have 2 GB of RAM, this barrier doesn't affect you, but where you decide to make the split may. If you choose a 2G/2G split, then theoretically the kernel can take up all 2GB of your memory (although highly unlikely). Setting it at 1G/3G caps how much system memory your kernel can use so you're guaranteed 1 GB for userspace programs.

Again, I'm not professing to be any sort of expert on this, so if I wrong please let me know .

[xvalentinex]

Thanks for the responses guys.