Linux Journal | December 2012 | Issue 224 - ACM Digital Library

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COLUMNSTHE OPEN-SOURCE CLASSROOMAs the Raspberry Pi doesn’t have a BIOS, theonly way to allocate RAM is to put a binary filenamed start.elf into the /boot directory.but nonetheless, the RPi does bootquite nicely from the included SDslot. Because it won’t boot fromUSB, installing an operating systemis easiest from a second computer.Several Linux distributions arecompiled for the ARM-based RPi,but the most popular by far is theDebian-based Raspbian OS.For best performance, a class 10 SDcard is recommended. Once you getthe SD card, instructions for copyingthe Raspbian OS are available athttp://www.raspberrypi.org. Thegood news is that even if your SDcard is small and slow, once the unitis booted, there’s no reason the OScan’t live largely on a USB drive. Ifyou want to experiment with movingthe bulk of the OS off the SD card,the Raspberry Pi Web site has somegreat tutorials.Splitting Your BrainThe Raspberry Pi comes with a sharedsystem for system RAM and GPU RAM.Because the GPU is a powerful devicecapable of driving a high-resolutionmonitor and hardware renderingseveral types of video, if you’re usingthe RPi in a multimedia setting, you’llwant to be sure the GPU gets plentyof RAM. If you’re running the Pi ina headless server mode, you’ll likelywant to use as little of the RAM aspossible for the GPU, reserving thebulk for the system.The absolute minimum the GPUneeds is 16MB. That doesn’t seemlike a lot, but because the systemmaxes out at 256MB or 512MB, even16MB can be significant. As theRaspberry Pi doesn’t have a BIOS,the only way to allocate RAM is toput a binary file named start.elfinto the /boot directory. Due to theproprietary nature of the GPU, thosevarious .elf files are downloadedfrom http://www.raspberrypi.org.If you’re running headless, you’llwant to grab the .elf binary withonly 16MB dedicated to GPU.Otherwise, you’ll have to decide howmuch GPU RAM your applicationwill require. Trial and error might bethe only way to figure out the bestsplit. The good news is that the newmodel B units come with 512MB of52 / DECEMBER 2012 / WWW.LINUXJOURNAL.COM
COLUMNSTHE OPEN-SOURCE CLASSROOMRAM instead of 256, so it allowsfor much more flexibility with thememory split.To Swap or Not to SwapSD cards are getting better everyyear when it comes to wear levelingand write cycles. The concern isalways that excessive use of theSD card will wear it out, and it willbecome read-only. Because the RPialso suffers from limited and nonexpandableRAM, the issue of swapspace is highly debated.Raspbian comes installed bydefault with a swap file active—not a swap partition, but a swapfile. Whether this swap file willwear out your SD card is still upfor debate. I tend to be paranoidabout such things, so I searched foran alternative. Because I have the512MB units, I have a little moreRAM to work with, so I decidedto use zRam to compress someof the system RAM and use it asswap space. How does that work?Basically, zRam allows the systemto use RAM as swap space, but in acompressed mode that allows it toappear as more RAM+swap than thesystem actually contains.zRam already is compiled into theRaspbian kernel, so if you want totry my method, it just takes a littlescripting. I absolutely love the scriptI found at the eXtremeSHOK.comWeb site. That site is off-line atthe time of this writing, so I’veincluded the script below. Createa file called /etc/init.d/zram, whichcontains the following:#!/bin/bash### BEGIN INIT INFO#Provides: zram#Required-Start:#Required-Stop:#Default-Start: 2 3 4 5#Default-Stop: 0 1 6#Short-Description: Increased Performance In Linux With zRam#(Virtual Swap Compressed in RAM)#Description: Adapted for Raspbian (Rasberry pi) by eXtremeSHOK.com#using https://raw.github.com/gionn/etc/master/init.d/zram### END INIT INFOstart() {mem_total_kb=$(grep MemTotal /proc/meminfo | grep -E➥--only-matching '[[:digit:]]+')modprobe zramsleep 1#only using 50% of system memory, comment the line#below to use 100% of system memorymem_total_kb=$((mem_total_kb/2))echo $((mem_total_kb * 1024)) > /sys/block/zram0/disksizeWWW.LINUXJOURNAL.COM / DECEMBER 2012 / 53
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Linux Journal | December 2012 | Issue 224 - ACM Digital Library

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