Re: [PATCH v8] mm: Proactive compaction
From: Nathan Chancellor <hidden>
Date: 2020-06-23 02:26:41
Also in:
linux-mips, linux-mm, lkml
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@dhcp22.suse.cz/ (local)
[3] https://lwn.net/Articles/817905/
Signed-off-by: Nitin Gupta <redacted>
Reviewed-by: Vlastimil Babka <redacted>
Reviewed-by: Khalid Aziz <redacted>
Reviewed-by: Oleksandr Natalenko <redacted>
Tested-by: Oleksandr Natalenko <redacted>
To: Andrew Morton <akpm@linux-foundation.org>
CC: Vlastimil Babka <redacted>
CC: Khalid Aziz <redacted>
CC: Michal Hocko <mhocko@suse.com>
CC: Mel Gorman <redacted>
CC: Matthew Wilcox <willy@infradead.org>
CC: Mike Kravetz <redacted>
CC: Joonsoo Kim <redacted>
CC: David Rientjes <rientjes@google.com>
CC: Nitin Gupta <redacted>
CC: Oleksandr Natalenko <redacted>
CC: linux-kernel <redacted>
CC: linux-mm <redacted>
CC: Linux API <redacted>
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 :)
Cheers,
Nathan