On 06/23/2020 10:26 AM, Nathan Chancellor wrote:
> On Tue, Jun 16, 2020 at 01:45:27PM -0700, Nitin Gupta wrote:
>> For some applications, we need to allocate almost all memory as
>> hugepages. However, on a running system, higher-order allocations can
>> fail if the memory is fragmented. Linux kernel currently does on-demand
>> compaction as we request more hugepages, but this style of compaction
>> incurs very high latency. Experiments with one-time full memory
>> compaction (followed by hugepage allocations) show that kernel is able
>> to restore a highly fragmented memory state to a fairly compacted memory
>> state within <1 sec for a 32G system. Such data suggests that a more
>> proactive compaction can help us allocate a large fraction of memory as
>> hugepages keeping allocation latencies low.
>>
>> For a more proactive compaction, the approach taken here is to define a
>> new sysctl called 'vm.compaction_proactiveness' which dictates bounds
>> for external fragmentation which kcompactd tries to maintain.
>>
>> The tunable takes a value in range [0, 100], with a default of 20.
>>
>> Note that a previous version of this patch [1] was found to introduce
>> too many tunables (per-order extfrag{low, high}), but this one reduces
>> them to just one sysctl. Also, the new tunable is an opaque value
>> instead of asking for specific bounds of "external fragmentation", which
>> would have been difficult to estimate. The internal interpretation of
>> this opaque value allows for future fine-tuning.
>>
>> Currently, we use a simple translation from this tunable to [low, high]
>> "fragmentation score" thresholds (low=100-proactiveness, high=low+10%).
>> The score for a node is defined as weighted mean of per-zone external
>> fragmentation. A zone's present_pages determines its weight.
>>
>> To periodically check per-node score, we reuse per-node kcompactd
>> threads, which are woken up every 500 milliseconds to check the same. If
>> a node's score exceeds its high threshold (as derived from user-provided
>> proactiveness value), proactive compaction is started until its score
>> reaches its low threshold value. By default, proactiveness is set to 20,
>> which implies threshold values of low=80 and high=90.
>>
>> This patch is largely based on ideas from Michal Hocko [2]. See also the
>> LWN article [3].
>>
>> Performance data
>> ================
>>
>> System: x64_64, 1T RAM, 80 CPU threads.
>> Kernel: 5.6.0-rc3 + this patch
>>
>> echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/enabled
>> echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/defrag
>>
>> Before starting the driver, the system was fragmented from a userspace
>> program that allocates all memory and then for each 2M aligned section,
>> frees 3/4 of base pages using munmap. The workload is mainly anonymous
>> userspace pages, which are easy to move around. I intentionally avoided
>> unmovable pages in this test to see how much latency we incur when
>> hugepage allocations hit direct compaction.
>>
>> 1. Kernel hugepage allocation latencies
>>
>> With the system in such a fragmented state, a kernel driver then
>> allocates as many hugepages as possible and measures allocation
>> latency:
>>
>> (all latency values are in microseconds)
>>
>> - With vanilla 5.6.0-rc3
>>
>> percentile latency
>> –––––––––– –––––––
>> 5 7894
>> 10 9496
>> 25 12561
>> 30 15295
>> 40 18244
>> 50 21229
>> 60 27556
>> 75 30147
>> 80 31047
>> 90 32859
>> 95 33799
>>
>> Total 2M hugepages allocated = 383859 (749G worth of hugepages out of
>> 762G total free => 98% of free memory could be allocated as hugepages)
>>
>> - With 5.6.0-rc3 + this patch, with proactiveness=20
>>
>> sysctl -w vm.compaction_proactiveness=20
>>
>> percentile latency
>> –––––––––– –––––––
>> 5 2
>> 10 2
>> 25 3
>> 30 3
>> 40 3
>> 50 4
>> 60 4
>> 75 4
>> 80 4
>> 90 5
>> 95 429
>>
>> Total 2M hugepages allocated = 384105 (750G worth of hugepages out of
>> 762G total free => 98% of free memory could be allocated as hugepages)
>>
>> 2. JAVA heap allocation
>>
>> In this test, we first fragment memory using the same method as for (1).
>>
>> Then, we start a Java process with a heap size set to 700G and request
>> the heap to be allocated with THP hugepages. We also set THP to madvise
>> to allow hugepage backing of this heap.
>>
>> /usr/bin/time
>> java -Xms700G -Xmx700G -XX:+UseTransparentHugePages -XX:+AlwaysPreTouch
>>
>> The above command allocates 700G of Java heap using hugepages.
>>
>> - With vanilla 5.6.0-rc3
>>
>> 17.39user 1666.48system 27:37.89elapsed
>>
>> - With 5.6.0-rc3 + this patch, with proactiveness=20
>>
>> 8.35user 194.58system 3:19.62elapsed
>>
>> Elapsed time remains around 3:15, as proactiveness is further increased.
>>
>> Note that proactive compaction happens throughout the runtime of these
>> workloads. The situation of one-time compaction, sufficient to supply
>> hugepages for following allocation stream, can probably happen for more
>> extreme proactiveness values, like 80 or 90.
>>
>> In the above Java workload, proactiveness is set to 20. The test starts
>> with a node's score of 80 or higher, depending on the delay between the
>> fragmentation step and starting the benchmark, which gives more-or-less
>> time for the initial round of compaction. As t he benchmark consumes
>> hugepages, node's score quickly rises above the high threshold (90) and
>> proactive compaction starts again, which brings down the score to the
>> low threshold level (80). Repeat.
>>
>> bpftrace also confirms proactive compaction running 20+ times during the
>> runtime of this Java benchmark. kcompactd threads consume 100% of one of
>> the CPUs while it tries to bring a node's score within thresholds.
>>
>> Backoff behavior
>> ================
>>
>> Above workloads produce a memory state which is easy to compact.
>> However, if memory is filled with unmovable pages, proactive compaction
>> should essentially back off. To test this aspect:
>>
>> - Created a kernel driver that allocates almost all memory as hugepages
>> followed by freeing first 3/4 of each hugepage.
>> - Set proactiveness=40
>> - Note that proactive_compact_node() is deferred maximum number of times
>> with HPAGE_FRAG_CHECK_INTERVAL_MSEC of wait between each check
>> (=> ~30 seconds between retries).
>>
>> [1] https://patchwork.kernel.org/patch/11098289/
>> [2] https://lore.kernel.org/linux-mm/20161230131412.GI13301@xxxxxxxxxxxxxx/
>> [3] https://lwn.net/Articles/817905/
>>
>> Signed-off-by: Nitin Gupta <nigupta@xxxxxxxxxx>
>> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>> Reviewed-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
>> Reviewed-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxx>
>> Tested-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxx>
>> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> CC: Vlastimil Babka <vbabka@xxxxxxx>
>> CC: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
>> CC: Michal Hocko <mhocko@xxxxxxxx>
>> CC: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>> CC: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>> CC: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> CC: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>> CC: David Rientjes <rientjes@xxxxxxxxxx>
>> CC: Nitin Gupta <ngupta@xxxxxxxxxxxxxx>
>> CC: Oleksandr Natalenko <oleksandr@xxxxxxxxxx>
>> CC: linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>
>> CC: linux-mm <linux-mm@xxxxxxxxx>
>> CC: Linux API <linux-api@xxxxxxxxxxxxxxx>
>
> This is now in -next and causes the following build failure:
>
> $ make -skj"$(nproc)" ARCH=mips CROSS_COMPILE=mipsel-linux- O=out/mipsel distclean malta_kvm_guest_defconfig mm/compaction.o
> In file included from include/linux/dev_printk.h:14,
> from include/linux/device.h:15,
> from include/linux/node.h:18,
> from include/linux/cpu.h:17,
> from mm/compaction.c:11:
> In function 'fragmentation_score_zone',
> inlined from '__compact_finished' at mm/compaction.c:1982:11,
> inlined from 'compact_zone' at mm/compaction.c:2062:8:
> include/linux/compiler.h:339:38: error: call to '__compiletime_assert_301' declared with attribute error: BUILD_BUG failed
> 339 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^
> include/linux/compiler.h:320:4: note: in definition of macro '__compiletime_assert'
> 320 | prefix ## suffix(); \
> | ^~~~~~
> include/linux/compiler.h:339:2: note: in expansion of macro '_compiletime_assert'
> 339 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^~~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
> 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> | ^~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:59:21: note: in expansion of macro 'BUILD_BUG_ON_MSG'
> 59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
> | ^~~~~~~~~~~~~~~~
> arch/mips/include/asm/page.h:70:30: note: in expansion of macro 'BUILD_BUG'
> 70 | #define HUGETLB_PAGE_ORDER ({BUILD_BUG(); 0; })
> | ^~~~~~~~~
> mm/compaction.c:66:32: note: in expansion of macro 'HUGETLB_PAGE_ORDER'
> 66 | #define COMPACTION_HPAGE_ORDER HUGETLB_PAGE_ORDER
> | ^~~~~~~~~~~~~~~~~~
> mm/compaction.c:1898:28: note: in expansion of macro 'COMPACTION_HPAGE_ORDER'
> 1898 | extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
> | ^~~~~~~~~~~~~~~~~~~~~~
> In function 'fragmentation_score_zone',
> inlined from 'kcompactd' at mm/compaction.c:1918:12:
> include/linux/compiler.h:339:38: error: call to '__compiletime_assert_301' declared with attribute error: BUILD_BUG failed
> 339 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^
> include/linux/compiler.h:320:4: note: in definition of macro '__compiletime_assert'
> 320 | prefix ## suffix(); \
> | ^~~~~~
> include/linux/compiler.h:339:2: note: in expansion of macro '_compiletime_assert'
> 339 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^~~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
> 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> | ^~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:59:21: note: in expansion of macro 'BUILD_BUG_ON_MSG'
> 59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
> | ^~~~~~~~~~~~~~~~
> arch/mips/include/asm/page.h:70:30: note: in expansion of macro 'BUILD_BUG'
> 70 | #define HUGETLB_PAGE_ORDER ({BUILD_BUG(); 0; })
> | ^~~~~~~~~
> mm/compaction.c:66:32: note: in expansion of macro 'HUGETLB_PAGE_ORDER'
> 66 | #define COMPACTION_HPAGE_ORDER HUGETLB_PAGE_ORDER
> | ^~~~~~~~~~~~~~~~~~
> mm/compaction.c:1898:28: note: in expansion of macro 'COMPACTION_HPAGE_ORDER'
> 1898 | extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
> | ^~~~~~~~~~~~~~~~~~~~~~
> In function 'fragmentation_score_zone',
> inlined from 'kcompactd' at mm/compaction.c:1918:12:
> include/linux/compiler.h:339:38: error: call to '__compiletime_assert_301' declared with attribute error: BUILD_BUG failed
> 339 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^
> include/linux/compiler.h:320:4: note: in definition of macro '__compiletime_assert'
> 320 | prefix ## suffix(); \
> | ^~~~~~
> include/linux/compiler.h:339:2: note: in expansion of macro '_compiletime_assert'
> 339 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^~~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
> 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> | ^~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:59:21: note: in expansion of macro 'BUILD_BUG_ON_MSG'
> 59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
> | ^~~~~~~~~~~~~~~~
> arch/mips/include/asm/page.h:70:30: note: in expansion of macro 'BUILD_BUG'
> 70 | #define HUGETLB_PAGE_ORDER ({BUILD_BUG(); 0; })
> | ^~~~~~~~~
> mm/compaction.c:66:32: note: in expansion of macro 'HUGETLB_PAGE_ORDER'
> 66 | #define COMPACTION_HPAGE_ORDER HUGETLB_PAGE_ORDER
> | ^~~~~~~~~~~~~~~~~~
> mm/compaction.c:1898:28: note: in expansion of macro 'COMPACTION_HPAGE_ORDER'
> 1898 | extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
> | ^~~~~~~~~~~~~~~~~~~~~~
> In function 'fragmentation_score_zone',
> inlined from 'kcompactd' at mm/compaction.c:1918:12:
> include/linux/compiler.h:339:38: error: call to '__compiletime_assert_301' declared with attribute error: BUILD_BUG failed
> 339 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^
> include/linux/compiler.h:320:4: note: in definition of macro '__compiletime_assert'
> 320 | prefix ## suffix(); \
> | ^~~~~~
> include/linux/compiler.h:339:2: note: in expansion of macro '_compiletime_assert'
> 339 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^~~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
> 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> | ^~~~~~~~~~~~~~~~~~
> include/linux/build_bug.h:59:21: note: in expansion of macro 'BUILD_BUG_ON_MSG'
> 59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
> | ^~~~~~~~~~~~~~~~
> arch/mips/include/asm/page.h:70:30: note: in expansion of macro 'BUILD_BUG'
> 70 | #define HUGETLB_PAGE_ORDER ({BUILD_BUG(); 0; })
> | ^~~~~~~~~
> mm/compaction.c:66:32: note: in expansion of macro 'HUGETLB_PAGE_ORDER'
> 66 | #define COMPACTION_HPAGE_ORDER HUGETLB_PAGE_ORDER
> | ^~~~~~~~~~~~~~~~~~
> mm/compaction.c:1898:28: note: in expansion of macro 'COMPACTION_HPAGE_ORDER'
> 1898 | extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
> | ^~~~~~~~~~~~~~~~~~~~~~
> make[3]: *** [scripts/Makefile.build:281: mm/compaction.o] Error 1
> make[3]: Target '__build' not remade because of errors.
> make[2]: *** [Makefile:1765: mm] Error 2
> make[2]: Target 'mm/compaction.o' not remade because of errors.
> make[1]: *** [Makefile:336: __build_one_by_one] Error 2
> make[1]: Target 'distclean' not remade because of errors.
> make[1]: Target 'malta_kvm_guest_defconfig' not remade because of errors.
> make[1]: Target 'mm/compaction.o' not remade because of errors.
> make: *** [Makefile:185: __sub-make] Error 2
> make: Target 'distclean' not remade because of errors.
> make: Target 'malta_kvm_guest_defconfig' not remade because of errors.
> make: Target 'mm/compaction.o' not remade because of errors.
>
> I am not sure why MIPS is special with its handling of hugepage support
> but I am far from a MIPS expert :)
it seems that both HUGETLB_PAGE and TRANSPARENT_HUGEPAGE are disabled with malta_kvm_guest_defconfig.
>
> Cheers,
> Nathan
>
