Re: [PATCH v8] mm: Proactive compaction

[Nathan Chancellor]

On Mon, Jun 22, 2020 at 09:32:12PM -0700, Nitin Gupta wrote:
> On 6/22/20 7:26 PM, 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
> >>   ââââââââââ âââââââ
> >> 	   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
> >>   ââââââââââ âââââââ
> >> 	   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 :)
> > 
> 
> Can you check if this patch fixes the compile error:
> 
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 45fd24a0ea0b..02963ffb9e70 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -62,7 +62,7 @@ static const unsigned int
> HPAGE_FRAG_CHECK_INTERVAL_MSEC = 500;
>   */
>  #if defined CONFIG_TRANSPARENT_HUGEPAGE
>  #define COMPACTION_HPAGE_ORDER HPAGE_PMD_ORDER
> -#elif defined HUGETLB_PAGE_ORDER
> +#elif defined CONFIG_HUGETLBFS
>  #define COMPACTION_HPAGE_ORDER HUGETLB_PAGE_ORDER
>  #else
>  #define COMPACTION_HPAGE_ORDER (PMD_SHIFT - PAGE_SHIFT)
> 
> 
> 
> 

Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> # build