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For a raspberrypi image, mainly there are two partitions, kernel and rootfs.  The size of kernel image size is 6M~48M.  Rootfs is mounted on  SD card and its value can be increased using gparted,fdisk or similar tool.  By default while flashing an  image to SD card, two partitions are created, namely mmcblk0p1 and mmcblk0p2. rootfs image is stored in mmcblk0p2 whereas mmcblk0p1 holds boot information.

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root@raspberrypi-rdk-hybrid:~# fdisk -l
Disk /dev/mmcblk0: 14 GB, 15552479232 bytes, 30375936 sectors
474624 cylinders, 4 heads, 16 sectors/track
Units: sectors of 1 * 512 = 512 bytes

Device Device                          Boot StartCHS    EndCHS        StartLBA     EndLBA    Sectors  Size Id EndLBA        Sectors      Size           Id Type
/dev/mmcblk0p1 *       64,0,1 1                   841,0,32          8192          107679           8192 99488      107679      99488 48.5M  c 5M    c Win95 FAT32 (LBA)
/dev/mmcblk0p2    896mmcblk0p2          896,0,1 1              1023 1023,3,32       114688       10711039    114688  10596352    10711039 5174M    10596352 5174M 83 Linux
root@raspberrypi-rdk-hybrid:~# 

For changing sd card partition using gparted refer below link.

         Partioning and Resizing using GParted GUI


Memory Usage 

      The memory in raspberrypi is partitioned into different sessions and the details of these partitions and allocated size for these partitions  can be found out using df command.  df -h command gives size allocated to each of these partitions and the size that is already used by different components.

...

ComponentSize
Kernel code9216K
RW data737K
RO data2928K
init1024K
BSS853K
vmalloc1097K
cma  total262K
KernelStack2536 kB
Shmem14596 kB
tmpfs95876k
devtmpfs347800k

...

Graphics memory 

   Graphics memory, commonly called GPU memory is the amount of memory available on graphics card and is used by graphics processor. We can use vcgencmd for getting gpu memory information.

Please refer below link for GPU memory details.

   GPU Memory Consumption Analysis

root@raspberrypi-rdk-hybrid:~# vcgencmd get_mem gpu
gpu=64M

Changing tmpfs partition size

   Size of tmpfs partition can be changed by using fstab. Below are the list of tmpfs partitions. 

...

The size of these partitions can be changed by by mentioning size in /etc/fstab.  For example, we reduce size of /run to ~100M by mentioning size as below.  Reboot the device after fstab is modified.

root@raspberrypi-rdk-hybrid:~# vi /etc/fstab
# stock fstab - you probably want to override this with a machine specific one

/dev/root            /                            auto       defaults              1  0
proc                     /proc                   proc         defaults              0  0
devpts               /dev/pts             devpts     mode=0620,ptmxmode=0666,gid=5      0  0
tmpfs                /run                       tmpfs      mode=0755,nodev,nosuid,strictatime,size=103741824 0  0
tmpfs                /var/volatile        tmpfs      defaults              0  0

# uncomment this if your device has a SD/MMC/Transflash slot
#/dev/mmcblk0p1       /media/card          auto       defaults,sync,noauto  0  0

/dev/mmcblk0p1    /boot    vfat    defaults    0    0

The changed size can be verified using df command.  The above change results in

root@raspberrypi-rdk-hybrid:~# df -h
Filesystem                            Size      Used          Available Use%   Mounted on
/dev/root                           865.5M    557.0M    247.0M       69%     /
devtmpfs                           339.6M         0          339.6M       0%       /dev
tmpfs                                  468.1M      8.0K      468.1M       0%      /dev/shm
tmpfs                                 98.9M      8.8M        90.2M         9%       /run
tmpfs                                468.1M         0          468.1M       0%      /sys/fs/cgroup
tmpfs                                468.1M      6.6M      461.5M      1%     /tmp
tmpfs                                468.1M      6.6M      461.5M      1%      /etc/snmp/snmpd.conf
tmpfs                                468.1M    428.0K    467.7M      0%      /var/volatile
/dev/root                        865.5M    557.0M    247.0M      69%   /var/lib
/dev/mmcblk0p1           48.5M     27.4M       21.0M       57%   /boot
tmpfs                               93.6M         0            93.6M         0%   /run/user/0