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Subject:  Re: How much RAM is "enough"? Date:  12/4/2012  2:40 PM
Author:  JeanDavid Number:  182779 of 188750

Are the people who go to 8 or even 16 GB doing something really advanced

In late 2003, I assembled a computer with a mother board that would take 2 Xeon processors, and had memory slots enough for 16 GBytes of RAM in it. I put in 4 BBytes. Years passed, price of RAM went down, and I stuck 4 GBytes more RAM in it before they stopped making that kind of RAM modules. Those Xeon processors could handle, I think, 256 GBytes of RAM, but there were not enough slots for it.

Now in a 32-bit machine, an OS like Windows could not handle more than about 4 GBytes per machine. I happen to run Red Hat Enterpise Linux (RHEL), and that can run up to 256 GBytes on a machine, but no single process gets more than 4 GBytes. Actually, the OS kernel can get all it wants because it has access to the memory management reisters, though it can see only 4 GBytes at any one time.

My present machine happens to be 4-bit, so I run a 64-bit version of RHEL 6.3, but from a user's point of view, that makes little difference.

So what Linux does is gives up to 4G to each process that needs it, and a coupla G to itself (last time I looked, which was not recently), and uses most of the rest for buffering. At the present time, I am not doing much, so my processor and memory usage looks something like this:

top - 14:29:35 up 5 days, 6:35, 2 users, load average: 4.18, 4.33, 4.31
Tasks: 257 total, 5 running, 252 sleeping, 0 stopped, 0 zombie
Cpu0 : 1.5%us, 0.9%sy, 97.6%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu1 : 0.0%us, 0.4%sy, 99.6%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu2 : 2.4%us, 1.3%sy, 96.3%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu3 : 0.0%us, 13.7%sy, 86.1%ni, 0.0%id, 0.0%wa, 0.2%hi, 0.0%si, 0.0%st
Mem: 7981712k total, 5352752k used, 2628960k free, 479736k buffers
Swap: 4095992k total, 1568k used, 4094424k free, 2847776k cached

The processors are fully loaded, though running at the nice level most of the time, because I have BOINC running in the background.

It is using about 2 Gigabytes for programs and their date, about 3 GB for buffering (unwritten output) and Caching (recently read input). It keeps a lot of recently read stuff because it might as well. Unused memory is wasted anyway, so they keep stuff around. If I exit Firefox, for example, it probably hangs around in the cache for quite a while, so if I need it again, it does not need to read it in from disk. The kernel can just copy it to where it is needed in a memory-to-memory copy, or if it is really lucky, just point the memory mapping registers to it.
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