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. David Rientjes 于2020年6月25日周四 上午1:22写道： > On Wed, 24 Jun 2020, 孙世龙 sunshilong wrote: > > > >> Are there still some methods that could be used by the Linux kernel > > >> to reduce memory fragmentation while both CONFIG-MIGRATION > > >> and CONFIG-COMPACTION are disabled? > > > > > > > > >We do have mobility grouping on pageblock order. > > >Also, I think you can use ZONE_MOVABLE without migration and compaction, > > >to at least locally limit unmovable fragmentation. > > It's a good news. > > > > Could you please explain that in more detail for me or suggest some > documents > > for me to go through. > > > > /proc/buddyinfo and /proc/pagetypeinfo will show the various pageblocks on > the system. 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. > > > As per the post( > http://lkml.iu.edu/hypermail/linux/kernel/1703.1/06782.html), > > I think it's something like ZONE_NORMAL. And I find that > > "ZONE_MOVEABLE" is available on Linux-v4.9. > > > > 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. > > > 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.