[PATCH v2 0/4] malloc: Improve Huge Page support
Adhemerval Zanella
adhemerval.zanella@linaro.org
Thu Aug 19 11:26:45 GMT 2021
On 18/08/2021 15:11, Siddhesh Poyarekar wrote:
> On 8/18/21 7:49 PM, Adhemerval Zanella wrote:
>> Linux currently supports two ways to use Huge Pages: either by using
>> specific flags directly with the syscall (MAP_HUGETLB for mmap(), or
>> SHM_HUGETLB for shmget()), or by using Transparent Huge Pages (THP)
>> where the kernel will try to move allocated anonymous pages to Huge
>> Pages blocks transparent to application.
>>
>> Also, THP current support three different modes [1]: 'never', 'madvise',
>> and 'always'. The 'never' is self-explanatory and 'always' will enable
>> THP for all anonymous memory. However, 'madvise' is still the default
>> for some systems and for such cases THP will be only used if the memory
>> range is explicity advertise by the program through a
>> madvise(MADV_HUGEPAGE) call.
>>
>> This patchset adds a two new tunables to improve malloc() support with
>> Huge Page:
>
> I wonder if this could be done with just the one tunable, glibc.malloc.hugepages where:
>
> 0: Disabled (default)
> 1: Transparent, where we emulate "always" behaviour of THP
> 2: HugeTLB enabled with default hugepage size
> <size>: HugeTLB enabled with the specified page size
I though about it, and decided to use two tunables because although
for mmap() system allocation both tunable are mutually exclusive
(since it does not make sense to madvise() a mmap(MAP_HUGETLB)
we still use sbrk() on main arena. The way I did for sbrk() is to align
to the THP page size advertisen by the kernel, so using the tunable
does change the behavior slightly (it is not 'transparent' as the
madvise call).
So to use only one tunable would require to either drop the sbrk()
madvise when MAP_HUGETLB is used, move it to another tunable (say
'3: HugeTLB enabled with default hugepage size and madvise() on sbrk()),
or assume it when huge pages should be used.
(and how do we handle sbrk() with explicit size?)
If one tunable is preferable I think it would be something like:
0: Disabled (default)
1: Transparent, where we emulate "always" behaviour of THP
sbrk() is also aligned to huge page size and issued madvise()
2: HugeTLB enabled with default hugepage size and sbrk() as
handled are 1
> <size>: HugeTLB enabled with the specified page size and sbrk()
are handled as 1
By forcing the sbrk() and madvise() on all tunables value make
the expectation to use huge pages in all possible occasions.
>
> When using HugeTLB, we don't really need to bother with THP so they seem mutually exclusive.
>
>>
>> - glibc.malloc.thp_madvise: instruct the system allocator to issue
>> a madvise(MADV_HUGEPAGE) call after a mmap() one for sizes larger
>> than the default huge page size. The default behavior is to
>> disable it and if the system does not support THP the tunable also
>> does not enable the madvise() call.
>>
>> - glibc.malloc.mmap_hugetlb: instruct the system allocator to round
>> allocation to huge page sizes along with the required flags
>> (MAP_HUGETLB for Linux). If the memory allocation fails, the
>> default system page size is used instead. The default behavior is
>> to disable and a value of 1 uses the default system huge page size.
>> A positive value larger than 1 means to use a specific huge page
>> size, which is matched against the supported ones by the system.
>>
>> The 'thp_madvise' tunable also changes the sbrk() usage by malloc
>> on main arenas, where the increment is now aligned to the huge page
>> size, instead of default page size.
>>
>> The 'mmap_hugetlb' aims to replace the 'morecore' removed callback
>> from 2.34 for libhugetlbfs (where the library tries to leverage the
>> huge pages usage instead to provide a system allocator). By
>> implementing the support directly on the mmap() code patch there is
>> no need to try emulate the morecore()/sbrk() semantic which simplifies
>> the code and make memory shrink logic more straighforward.
>>
>> The performance improvements are really dependent of the workload
>> and the platform, however a simple testcase might show the possible
>> improvements:
>
> A simple test like below in benchtests would be very useful to at least get an initial understanding of the behaviour differences with different tunable values. Later those who care can add more relevant workloads.
Yeah, I am open to suggestions on how to properly test it. The issue
is we need to have specific system configuration either by proper
kernel support (THP) or with reserved large pages to actually test
it.
For THP the issue is really 'transparent' for user, which means that
we will need to poke on specific Linux sysfs information to check if
huge pages are being used. And we might not get the expected answer
depending of the system load and memory utilization (the advised
pages might not be moved to large pages if there is no sufficient
memory).
>
>>
>> $ cat hugepages.cc
>> #include <unordered_map>
>>
>> int
>> main (int argc, char *argv[])
>> {
>> std::size_t iters = 10000000;
>> std::unordered_map <std::size_t, std::size_t> ht;
>> ht.reserve (iters);
>> for (std::size_t i = 0; i < iters; ++i)
>> ht.try_emplace (i, i);
>>
>> return 0;
>> }
>> $ g++ -std=c++17 -O2 hugepages.cc -o hugepages
>>
>> On a x86_64 (Ryzen 9 5900X):
>>
>> Performance counter stats for 'env
>> GLIBC_TUNABLES=glibc.malloc.thp_madvise=0 ./testrun.sh ./hugepages':
>>
>> 98,874 faults
>> 717,059 dTLB-loads
>> 411,701 dTLB-load-misses # 57.42% of all dTLB
>> cache accesses
>> 3,754,927 cache-misses # 8.479 % of all
>> cache refs
>> 44,287,580 cache-references
>>
>> 0.315278378 seconds time elapsed
>>
>> 0.238635000 seconds user
>> 0.076714000 seconds sys
>>
>> Performance counter stats for 'env
>> GLIBC_TUNABLES=glibc.malloc.thp_madvise=1 ./testrun.sh ./hugepages':
>>
>> 1,871 faults
>> 120,035 dTLB-loads
>> 19,882 dTLB-load-misses # 16.56% of all dTLB
>> cache accesses
>> 4,182,942 cache-misses # 7.452 % of all
>> cache refs
>> 56,128,995 cache-references
>>
>> 0.262620733 seconds time elapsed
>>
>> 0.222233000 seconds user
>> 0.040333000 seconds sys
>>
>>
>> On an AArch64 (cortex A72):
>>
>> Performance counter stats for 'env
>> GLIBC_TUNABLES=glibc.malloc.thp_madvise=0 ./testrun.sh ./hugepages':
>>
>> 98835 faults
>> 2007234756 dTLB-loads
>> 4613669 dTLB-load-misses # 0.23% of all dTLB
>> cache accesses
>> 8831801 cache-misses # 0.504 % of all
>> cache refs
>> 1751391405 cache-references
>>
>> 0.616782575 seconds time elapsed
>>
>> 0.460946000 seconds user
>> 0.154309000 seconds sys
>>
>> Performance counter stats for 'env
>> GLIBC_TUNABLES=glibc.malloc.thp_madvise=1 ./testrun.sh ./hugepages':
>>
>> 955 faults
>> 1787401880 dTLB-loads
>> 224034 dTLB-load-misses # 0.01% of all dTLB
>> cache accesses
>> 5480917 cache-misses # 0.337 % of all
>> cache refs
>> 1625937858 cache-references
>>
>> 0.487773443 seconds time elapsed
>>
>> 0.440894000 seconds user
>> 0.046465000 seconds sys
>>
>>
>> And on a powerpc64 (POWER8):
>>
>> Performance counter stats for 'env
>> GLIBC_TUNABLES=glibc.malloc.thp_madvise=0 ./testrun.sh ./hugepages
>> ':
>>
>> 5453 faults
>> 9940 dTLB-load-misses
>> 1338152 cache-misses # 0.101 % of all
>> cache refs
>> 1326037487 cache-references
>>
>> 1.056355887 seconds time elapsed
>>
>> 1.014633000 seconds user
>> 0.041805000 seconds sys
>>
>> Performance counter stats for 'env
>> GLIBC_TUNABLES=glibc.malloc.thp_madvise=1 ./testrun.sh ./hugepages
>> ':
>>
>> 1016 faults
>> 1746 dTLB-load-misses
>> 399052 cache-misses # 0.030 % of all
>> cache refs
>> 1316059877 cache-references
>>
>> 1.057810501 seconds time elapsed
>>
>> 1.012175000 seconds user
>> 0.045624000 seconds sys
>>
>> It is worth to note that the powerpc64 machine has 'always' set
>> on '/sys/kernel/mm/transparent_hugepage/enabled'.
>>
>> Norbert Manthey's paper has more information with a more thoroughly
>> performance analysis.
>>
>> For testing run make check on x86_64-linux-gnu with thp_pagesize=1
>> (directly on ptmalloc_init() after tunable initialiazation) and
>> with mmap_hugetlb=1 (also directly on ptmalloc_init()) with about
>> 10 large pages (so the fallback mmap() call is used) and with
>> 1024 large pages (so all mmap(MAP_HUGETLB) are successful).
>
> You could add tests similar to mcheck and malloc-check, i.e. add $(tests-hugepages) to run all malloc tests again with the various tunable values. See tests-mcheck for example.
Ok, I can work with this. This might not add much if the system is
not configured with either THP or with some huge page pool but at
least adds some coverage.
>
>> --
>>
>> Changes from previous version:
>>
>> - Renamed thp_pagesize to thp_madvise and make it a boolean state.
>> - Added MAP_HUGETLB support for mmap().
>> - Remove system specific hooks for THP huge page size in favor of
>> Linux generic implementation.
>> - Initial program segments need to be page aligned for the
>> first madvise call.
>>
>> Adhemerval Zanella (4):
>> malloc: Add madvise support for Transparent Huge Pages
>> malloc: Add THP/madvise support for sbrk
>> malloc: Move mmap logic to its own function
>> malloc: Add Huge Page support for sysmalloc
>>
>> NEWS | 9 +-
>> elf/dl-tunables.list | 9 +
>> elf/tst-rtld-list-tunables.exp | 2 +
>> include/libc-pointer-arith.h | 10 +
>> malloc/arena.c | 7 +
>> malloc/malloc-internal.h | 1 +
>> malloc/malloc.c | 263 +++++++++++++++------
>> manual/tunables.texi | 23 ++
>> sysdeps/generic/Makefile | 8 +
>> sysdeps/generic/malloc-hugepages.c | 37 +++
>> sysdeps/generic/malloc-hugepages.h | 49 ++++
>> sysdeps/unix/sysv/linux/malloc-hugepages.c | 201 ++++++++++++++++
>> 12 files changed, 542 insertions(+), 77 deletions(-)
>> create mode 100644 sysdeps/generic/malloc-hugepages.c
>> create mode 100644 sysdeps/generic/malloc-hugepages.h
>> create mode 100644 sysdeps/unix/sysv/linux/malloc-hugepages.c
>>
>
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