[PATCH v4 0/4] mm/filemap: optimize folio adding and splitting

Kairui Song
2024-04-16T07:17:46Z

Currently, at least 3 tree walks are needed for filemap folio adding if
the folio is previously evicted. One for getting the order of current slot,
one for ranged conflict check, and one for another order retrieving.
If a split is needed, more walks are needed.

This series is trying to merge these walks, and speed up filemap_add_folio,
I see a 7.5% - 12.5% performance gain for fio stress test.

So instead of doing multiple tree walks, do one optimism range check
with lock hold, and exit if raced with another insertion. If a shadow
exists, check it with a new xas_get_order helper before releasing the
lock to avoid redundant tree walks for getting its order.

Drop the lock and do the allocation only if a split is needed.

In the best case, it only need to walk the tree once. If it needs
to alloc and split, 3 walks are issued (One for first ranged
conflict check and order retrieving, one for the second check after
allocation, one for the insert after split).

Testing with 4K pages, in an 8G cgroup, with 16G brd as block device:

echo 3 > /proc/sys/vm/drop_caches

fio -name=cached --numjobs=16 --filename=/mnt/test.img \
--buffered=1 --ioengine=mmap --rw=randread --time_based \
--ramp_time=30s --runtime=5m --group_reporting

Before:
bw ( MiB/s): min= 1027, max= 3520, per=100.00%, avg=2445.02, stdev=18.90, samples=8691
iops : min=263001, max=901288, avg=625924.36, stdev=4837.28, samples=8691

After (+7.3%):
bw ( MiB/s): min= 493, max= 3947, per=100.00%, avg=2625.56, stdev=25.74, samples=8651
iops : min=126454, max=1010681, avg=672142.61, stdev=6590.48, samples=8651

Test result with THP (do a THP randread then switch to 4K page in hope it
issues a lot of splitting):

echo 3 > /proc/sys/vm/drop_caches

fio -name=cached --numjobs=16 --filename=/mnt/test.img \
--buffered=1 --ioengine=mmap -thp=1 --readonly \
--rw=randread --time_based --ramp_time=30s --runtime=10m \
--group_reporting

fio -name=cached --numjobs=16 --filename=/mnt/test.img \
--buffered=1 --ioengine=mmap \
--rw=randread --time_based --runtime=5s --group_reporting

Before:
bw ( KiB/s): min= 4141, max=14202, per=100.00%, avg=7935.51, stdev=96.85, samples=18976
iops : min= 1029, max= 3548, avg=1979.52, stdev=24.23, samples=18976·

READ: bw=4545B/s (4545B/s), 4545B/s-4545B/s (4545B/s-4545B/s), io=64.0KiB (65.5kB), run=14419-14419msec

After (+10.4%):
bw ( KiB/s): min= 4611, max=15370, per=100.00%, avg=8928.74, stdev=105.17, samples=19146
iops : min= 1151, max= 3842, avg=2231.27, stdev=26.29, samples=19146

READ: bw=4635B/s (4635B/s), 4635B/s-4635B/s (4635B/s-4635B/s), io=64.0KiB (65.5kB), run=14137-14137msec

The performance is better for both 4K (+7.5%) and THP (+12.5%) cached read.

V3: https://lore.kernel.org/linux-mm/20240415171857.19244-1-ryncsn@gmail.com/
Updates from V3:
- Simplify comment, and fold in a sparse warning fix, as
suggested by Matthew Wilcox.

V2: https://lore.kernel.org/lkml/20240325171405.99971-1-ryncsn@gmail.com/
Updates from V2:
- Fix the misusage of locks in test module:
https://lore.kernel.org/oe-lkp/202404151046.448e2d6e-lkp@intel.com

V1: https://lore.kernel.org/lkml/20240319092733.4501-1-ryncsn@gmail.com/
Updates from V1:
- Collect Acks.
- Add tests for new xas_get_order and combined usage of xas_get_order with
xas_for_each_conflict.
- Fix a memleak for patch 4/4 and modify the function in place instead
of adding a new helper.
- Update benchmark, I forgot to drop cache and disable THP for pervious
test, so the result was for mixed usaged of split and add. The result
is even better now.

Kairui Song (4):
mm/filemap: return early if failed to allocate memory for split
mm/filemap: clean up hugetlb exclusion code
lib/xarray: introduce a new helper xas_get_order
mm/filemap: optimize filemap folio adding

include/linux/xarray.h | 6 +++
lib/test_xarray.c | 93 ++++++++++++++++++++++++++++++++++++++++++
lib/xarray.c | 49 ++++++++++++++--------
mm/filemap.c | 74 +++++++++++++++++++++------------
4 files changed, 179 insertions(+), 43 deletions(-)

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2.44.0