RAM split between lowmem and highmem

Question

I have compared /proc/meminfo for a galaxys2 (arm exynos4) device running Android gingerbread and ice cream sandwich (cyanogen CM9). I noticed that the kernel splits the memory differently between low memory and high memory:

For ICS/CM9 (3.0 kernel):

cat /proc/meminfo:

MemTotal:         843624 kB
MemFree:           68720 kB
Buffers:            1532 kB
Cached:           115720 kB
SwapCached:            0 kB
Active:           487780 kB
Inactive:          64524 kB
Active(anon):     436316 kB
Inactive(anon):     1764 kB
Active(file):      51464 kB
Inactive(file):    62760 kB
Unevictable:         748 kB
Mlocked:               0 kB
**HighTotal:        278528 kB**
HighFree:          23780 kB
**LowTotal:         565096 kB**
LowFree:           44940 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 4 kB
Writeback:             0 kB
AnonPages:        435848 kB
Mapped:            45364 kB
Shmem:              2276 kB
Slab:              37996 kB
SReclaimable:      10028 kB
SUnreclaim:        27968 kB
KernelStack:       10064 kB
PageTables:        16688 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:      421812 kB
Committed_AS:    8549052 kB
VmallocTotal:     188416 kB
VmallocUsed:      104480 kB
VmallocChunk:      26500 kB

For GB (2.6 kernel):

cat /proc/meminfo:

MemTotal:         856360 kB
MemFree:           22264 kB
Buffers:           57000 kB
Cached:           337320 kB
SwapCached:            0 kB
Active:           339064 kB
Inactive:         379148 kB
Active(anon):     212928 kB
Inactive(anon):   112964 kB
Active(file):     126136 kB
Inactive(file):   266184 kB
Unevictable:         396 kB
Mlocked:               0 kB
**HighTotal:        462848 kB**
HighFree:           1392 kB
**LowTotal:         393512 kB**
LowFree:           20872 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 4 kB
Writeback:             0 kB
AnonPages:        324312 kB
Mapped:            97092 kB
Shmem:              1580 kB
Slab:              29160 kB
SReclaimable:      13924 kB
SUnreclaim:        15236 kB
KernelStack:        8352 kB
PageTables:        23828 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:      428180 kB
Committed_AS:    4001404 kB
VmallocTotal:     196608 kB
VmallocUsed:      104804 kB
VmallocChunk:      57092 kB

I have noticed that on the 3.0 kernel memory pressure is evident and the out of memory killer is invoked frequently.

I have two questions regarding this:

1. Is is possible that in the 3.0 layout (less highmem more lowmem) applications have less available memory? Could that explain the high memory pressure?
2. Is it possible to change the layout in the 3.0 kernel in order to make it more similar to the 2.6 layout (i.e. more highmem less lowmem)?

Answer 1

As far as I recall, the split between high and low memory is a compilation parameter of the Kernel, so it should be possible to set it differently (at compile time). I do not know why so much is given to the high memory region on your examples. On x86 with 1 GB physical RAM it is about 896 MB for low memory and 128 MB is high memory.

It would seem that Android needs more high memory than a typical 32 bit x86 desktop, I do not know which feature(s) of the Android echo-system would bring such requirements, so hopefully somebody else can tell you that.

You could try to investigate the memory zones to try to see what is the difference between Android ICS and GB. Simply do a cat /proc/zoneinfo. You can find some background information on these zones in this article, although take care that it was described for x86 arch.