Hi,David Rientjes, David Hildenbrand
Thanks a lot to both of you.
>Grouping pages by mobility simply means that we attempt (best
>effort) to allocate unmovable pages from the same pageblocks and movable
>pages from the same pageblocks.
How can I understand it in the right way, especially for "from the same
pageblocks"? Could you please explain that in more detail?
>Yes, you can use kernelcore= (or movablecore=) on the kernel command line
>to set this up: allocations from this zone must have __GFP_MOVABLE so
>they "should" be migratable. This is typically only useful when
>CONFIG_COMPACTION is enabled, however, to do that defragmentation work so
>that higher order memory becomes available.
I was very hopeful for this method(i.e. ZONE_MOVABLE).
It's really bad news. As the subject, I have no choice other than disabling
these options(i.e. CONFIG-MIGRATION and CONFIG-COMPACTION)
since I am using a real-time OS(i.e. it needs a patch to the Linux kernel
and some kconfig options should be disabled).
Are there still some methods that could be used by the Linux kernel
to reduce memory fragmentation?
>> Is there some potential problems that I should be aware of if I enable
>> "ZONE_MOVABLE" on real-time system?
>>
>Yes, if ZONE_MOVABLE is made too big then you can risk out of memory
>kills: movable allocations can fallback to ZONE_NORMAL but unmovable
>allocations cannot graduate to ZONE_MOVABLE. So if ZONE_NORMAL is full
>(either because you have too much unmovable memory in-use or too much
>movable fell back to ZONE_NORMAL), and you have more unmovable
>allocations, you'll get reclaim in ZONE_NORMAL and, at worst case, oom
>kills.
I can draw the conclusion that ZONE_NORMAL is only used to allocate
unmovable memory(i.e. no movable memory could be allocated from
ZONE_NORMAL) while "kernelcore= (or movablecore=)" option is set.
Am I right?
As the kernel without such option(i.e. kernelcore= or movablecore=), there are
both movable memory and unmovable memory in ZONE_NORMAL.
For details, see the footnote.
It's different with the option and without the option:
Only unmovable memory could be allocated from ZONE_NORMAL while the option
is set whereas both unmovable and movable could be allocated from it without
setting the option.
Am I right?
Here is part of the output when executing "sudo cat /proc/pagetypeinfo" on the platform
without the option(i.e. kernelcore= or movablecore=)：
Free pages count per migrate type at order
0 1 2 3 4 5 6 7 8 9 10
DMA, type Unmovable 0 1 1 0 2 1 1 0 1 0 0
DMA, type Movable 0 0 0 0 0 0 0 0 0 1 3
DMA, type Reclaimable 0 0 0 0 0 0 0 0 0 0 0
DMA, type HighAtomic 0 0 0 0 0 0 0 0 0 0 0
DMA, type Isolate 0 0 0 0 0 0 0 0 0 0 0
DMA32, type Unmovable 1 0 0 0 0 0 1 1 1 1 0
DMA32, type Movable 8 6 7 5 5 4 6 5 2 2 723
DMA32, type Reclaimable 0 0 0 0 0 0 0 0 0 0 0
DMA32, type HighAtomic 0 0 0 0 0 0 0 0 0 0 0
DMA32, type Isolate 0 0 0 0 0 0 0 0 0 0 0
Normal, type Unmovable 0 23 9 2 1 1 0 1 10 11 0
Normal, type Movable 1 791 767 630 111 11 5 5 2 0 929
Normal, type Reclaimable 0 2 4 2 2 1 1 1 1 1 0
Normal, type HighAtomic 0 0 0 0 0 0 0 0 0 0 0
Normal, type Isolate 0 0 0 0 0 0 0 0 0 0 0
Number of blocks type Unmovable Movable Reclaimable HighAtomic Isolate
Node 0, zone DMA 1 7 0 0 0
Node 0, zone DMA32 2 1526 0 0 0
Node 0, zone Normal 160 2318 74 0 0
Best regards.