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[SOLVED] Debian, Gentoo show different amounts RAM, # cpus
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ElleStone
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Joined: 12 Mar 2013
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PostPosted: Tue Apr 02, 2013 5:12 pm    Post subject: [SOLVED] Debian, Gentoo show different amounts RAM, # cpus Reply with quote

I installed a very basic Gentoo on my Acer Aspire 5570Z laptop, plus a very basic Debian Sid (minimal net install + xorg/icewm + some utility programs) on the same laptop, plus Kubuntu.

When doing "startx" to log into Icewm, then starting xterm and running top, Debian and Gentoo show different amounts of RAM (Kubuntu numbers are between the Debian and Gentoo numbers):

Debian: top 1024576 total, 107324 used, 917252 free, 7756 buffers, 70 tasks, 2 users
Gentoo: top 899676 total, 92160 used, 807516 free, 15928 buffers, 59 tasks, 2 users

Why does Gentoo show less system RAM than Debian? Did I do something wrong when installing Gentoo?

Also, in Gentoo, "cat /proc/cpuinfo" shows 1 processor with 2 cores, whereas Debian and Kubuntu show 2 processors, each with 2 cores (which I think can't possibly be true on my lowly little laptop). Does anyone know why such strange results?

Elle


Last edited by ElleStone on Tue Apr 02, 2013 7:43 pm; edited 1 time in total
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Ant P.
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PostPosted: Tue Apr 02, 2013 5:26 pm    Post subject: Reply with quote

To fix the RAM problem you need to enable 4GB highmem in the kernel, or use a patched kernel that allows changing the VM split from 896MB to 1GB.
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limn
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PostPosted: Tue Apr 02, 2013 5:43 pm    Post subject: Reply with quote

You also need a kernel with symmetric multi-processing support. You have an Intel® Pentium® Dual-Core processor.
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ElleStone
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PostPosted: Tue Apr 02, 2013 7:42 pm    Post subject: Reply with quote

Ant P and limn, thanks very much. I recompiled the kernel per your suggestions and Gentoo "top" now reports 1019792 kb total memory, very close to what Debian reports. Also, Gentoo "cat /proc/cpuinfo" reports two processors, each with two cores, which is mystifying because I'm pretty sure there's just one processor with two cores. But at least Gentoo and Debian are reporting the same things.

Regarding the "Symmetric multi-processing support" option, I had somehow overlooked the statement that "In multi-core systems, each core counts as one processor" (Gentoo handbook).

Regarding the "High Memory Support", I only have 1GB ram, so I didn't think it applied. But according to http://www.gentoo.org/doc/en/kernel-config.xml, "Due to limitations in the 32-bit address space of the x86 architecture, a kernel with default configuration can only support up to 896mb RAM. If your system has more memory, only the first 896mb will be visible, unless you enable high memory support."

I am marking this question as solved, but I do have a related question: When configuring the kernel, what is the difference between enabling pae support and enabling high memory support (which seems to enable pae support of itself)?

Thanks again, very much, to both of you,
Elle
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limn
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PostPosted: Tue Apr 02, 2013 10:25 pm    Post subject: Reply with quote

If you have hyper-threading enabled you have a two virtual cores for each physical one. These appear to kernel as CPU's. A dual core appears as 4, a quad core as 8.

In kernel 3.7.10 enabling "High Memory Support (4GB)" does not enable PAE. Enabling "High Memory Support (64GB)" automatically does. You would need PAE if you have more than 4Gb of RAM and wanted to access it.

You can have PAE enabled if you do not have more than 4Gb and you will not notice any difference.
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