[PATCH v2 00/16] ext4: better scalability for ext4 block allocation

Baokun Li
2025-06-23T07:46:57Z

Changes since v1:
* Patch 1: Prioritize checking if a group is busy to avoid unnecessary
checks and buddy loading. (Thanks to Ojaswin for the suggestion!)
* Patch 4: Using multiple global goals instead of moving the goal to the
inode level. (Thanks to Honza for the suggestion!)
* Collect RVB from Jan Kara and Ojaswin Mujoo.(Thanks for your review!)
* Add patch 2,3,7-16.
* Due to the change of test server, the relevant test data was refreshed.

v1: https://lore.kernel.org/r/20250523085821.1329392-1-libaokun@huaweicloud.com

Since servers have more and more CPUs, and we're running more containers
on them, we've been using will-it-scale to test how well ext4 scales. The
fallocate2 test (append 8KB to 1MB, truncate to 0, repeat) run concurrently
on 64 containers revealed significant contention in block allocation/free,
leading to much lower aggregate fallocate OPS compared to a single
container (see below).

1 | 2 | 4 | 8 | 16 | 32 | 64
-------|--------|--------|--------|--------|--------|-------
295287 | 70665 | 33865 | 19387 | 10104 | 5588 | 3588

Under this test scenario, the primary operations are block allocation
(fallocate) and block deallocation (truncate). The main bottlenecks for
these operations are the group lock and s_md_lock. Therefore, this patch
series primarily focuses on optimizing the code related to these two locks.

The following is a brief overview of the patches, see the patches for
more details.

Patch 1: Add ext4_try_lock_group() to skip busy groups to take advantage
of the large number of ext4 groups.

Patches 2-4: Split stream allocation's global goal into multiple goals and
protect them with memory barriers instead of the expensive s_md_lock.

Patches 5-6: minor cleanups

Patches 7: Converted s_mb_free_pending to atomic_t and used memory barriers
for consistency, instead of relying on the expensive s_md_lock.

Patches 8: When inserting free extents, we now attempt to merge them with
already inserted extents first, to reduce s_md_lock contention.

Patches 9: Updates bb_avg_fragment_size_order to -1 when a group is out of
free blocks, eliminating efficiency-impacting "zombie groups."

Patches 10: Fix potential largest free orders lists corruption when the
mb_optimize_scan mount option is switched on or off.

Patches 11-16: Convert mb_optimize_scan's existing unordered list traversal
to an ordered xarray, thereby reducing contention between block allocation
and freeing, similar to linear traversal.

"kvm-xfstests -c ext4/all -g auto" has been executed with no new failures.

Here are some performance test data for your reference:

Test: Running will-it-scale/fallocate2 on CPU-bound containers.
Observation: Average fallocate operations per container per second.

CPU: Kunpeng 920 | P80 | P1 |
Memory: 512GB |-------------------------|-------------------------|
Disk: 960GB SSD | base | patched | base | patched |
-------------------|-------|-----------------|-------|-----------------|
mb_optimize_scan=0 | 2667 | 20619 (+673.1%)| 314065| 299238 (-4.7%) |
mb_optimize_scan=1 | 2643 | 20119 (+661.2%)| 316344| 315268 (-0.3%) |

CPU: AMD 9654 * 2 | P96 | P1 |
Memory: 1536GB |-------------------------|-------------------------|
Disk: 960GB SSD | base | patched | base | patched |
-------------------|-------|-----------------|-------|-----------------|
mb_optimize_scan=0 | 3450 | 51983 (+1406.7%)| 205851| 207033 (+0.5%) |
mb_optimize_scan=1 | 3209 | 48486 (+1410.9%)| 207373| 202415 (-2.3%) |

Tests also evaluated this patch set's impact on fragmentation: a minor
increase in free space fragmentation for multi-process workloads, but a
significant decrease in file fragmentation:

Test Script：
```shell
#!/bin/bash

dir="/tmp/test"
disk="/dev/sda"

mkdir -p $dir

for scan in 0 1 ; do
mkfs.ext4 -F -E lazy_itable_init=0,lazy_journal_init=0 \
-O orphan_file $disk 200G
mount -o mb_optimize_scan=$scan $disk $dir

fio -directory=$dir -direct=1 -iodepth 128 -thread -ioengine=falloc \
-rw=write -bs=4k -fallocate=none -numjobs=64 -file_append=1 \
-size=1G -group_reporting -name=job1 -cpus_allowed_policy=split

e2freefrag $disk
e4defrag -c $dir # Without the patch, this could take 5-6 hours.
filefrag ${dir}/job* | awk '{print $2}' | \
awk '{sum+=$1} END {print sum/NR}'
umount $dir
done
```

Test results:
-------------------------------------------------------------|
| base | patched |
-------------------------|--------|--------|--------|--------|
mb_optimize_scan | linear |opt_scan| linear |opt_scan|
-------------------------|--------|--------|--------|--------|
bw(MiB/s) | 217 | 217 | 5718 | 5626 |
-------------------------|-----------------------------------|
Avg. free extent size(KB)| 1943732| 1943732| 1316212| 1171208|
Num. free extent | 71 | 71 | 105 | 118 |
-------------------------------------------------------------|
Avg. extents per file | 261967 | 261973 | 588 | 570 |
Avg. size per extent(KB) | 4 | 4 | 1780 | 1837 |
Fragmentation score | 100 | 100 | 2 | 2 |
-------------------------------------------------------------|

Comments and questions are, as always, welcome.

Thanks,
Baokun

Baokun Li (16):
ext4: add ext4_try_lock_group() to skip busy groups
ext4: remove unnecessary s_mb_last_start
ext4: remove unnecessary s_md_lock on update s_mb_last_group
ext4: utilize multiple global goals to reduce contention
ext4: get rid of some obsolete EXT4_MB_HINT flags
ext4: fix typo in CR_GOAL_LEN_SLOW comment
ext4: convert sbi->s_mb_free_pending to atomic_t
ext4: merge freed extent with existing extents before insertion
ext4: fix zombie groups in average fragment size lists
ext4: fix largest free orders lists corruption on mb_optimize_scan
switch
ext4: factor out __ext4_mb_scan_group()
ext4: factor out ext4_mb_might_prefetch()
ext4: factor out ext4_mb_scan_group()
ext4: convert free group lists to ordered xarrays
ext4: refactor choose group to scan group
ext4: ensure global ordered traversal across all free groups xarrays

fs/ext4/balloc.c | 2 +-
fs/ext4/ext4.h | 45 +-
fs/ext4/mballoc.c | 898 +++++++++++++++++++++---------------
fs/ext4/mballoc.h | 18 +-
include/trace/events/ext4.h | 3 -
5 files changed, 553 insertions(+), 413 deletions(-)

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2.46.1