EXT4 IOPS degradation in 5.10 compared to 5.4

Bhatnagar, Rishabh
2023-01-12T02:06:48Z

Hi Theodore/Jan

Please ignore the previous mail which isn't formatted well.

We have been seeing a consistent 3% degradation in IOPS score between
5.4 and 5.10 stable kernels while running fio tests.

I'm running test case on m6g.8xlarge AWS instances using arm64. The test
involves:

1. Creating 100GB volume with IO1 500 iops. Attaching it to the instance.

2. Setup and mount fs:

mke2fs -m 1 -t ext4 -b 4096 -L /mnt /dev/nvme1n1
mount -t ext4 -o noatime,nodiratime,data=ordered /dev/nvme1n1 /mnt

3. Install fio package and run following test:
(running 16 threads doing random buffered 16kb writes on a file.
ioengine=psync, runtime=60secs)

jobs=16
blocksize="16k"
filesize=1000000

if [[ -n $1 ]]; then jobs=$1; fi
if [[ -n $2 ]]; then blocksize=$2; fi

/usr/bin/fio --name=fio-test --directory=/mnt --rw=randwrite
--ioengine=psync --buffered=1 --bs=${blocksize} \
--max-jobs=${jobs} --numjobs=${jobs} --runtime=30 --thread \
--filename=file0 --filesize=${filesize} \
--fsync=1 --group_reporting --create_only=1 > /dev/null

sudo echo 1 > /proc/sys/vm/drop_caches

set -x
echo "Running with jobs=${jobs} filesize=${filesize}
blocksize=${blocksize}"
/usr/bin/fio --name=fio-test --directory=/mnt --rw=randwrite
--ioengine=psync --buffered=1 --bs=${blocksize} \
--max-jobs=${jobs} --numjobs=${jobs} --runtime=60 --thread \
--filename=file0 --filesize=${filesize} \
--fsync=1 --group_reporting --time_based

After doing some kernel bisecting between we were able to pinpoint this
commit that drops the iops score by 10~15 points (~3%).
ext4: avoid unnecessary transaction starts during writeback
(6b8ed62008a49751fc71fefd2a4f89202a7c2d4d)

We see higher iops/bw/total io after reverting the commit compared to
base 5.10 kernel.
Although the average clat is higher after reverting the commit the
higher bw drives the iops score higher as seen in below fio output.

Fio output (5.10.162):
write: io=431280KB, bw=7186.3KB/s, iops=449, runt= 60015msec
clat (usec): min=6, max=25942, avg=267.76,stdev=1604.25
lat (usec): min=6, max=25943, avg=267.93,stdev=1604.25
clat percentiles (usec):
| 1.00th=[ 9], 5.00th=[ 10], 10.00th=[ 16], 20.00th=[ 24]
| 30.00th=[ 34], 40.00th=[ 45], 50.00th=[ 58], 60.00th=[ 70],
| 70.00th=[ 81], 80.00th=[ 94], 90.00th=[ 107], 95.00th=[ 114],
| 99.00th=[10048], 99.50th=[14016], 99.90th=[20096], 99.95th=[21888],
| 99.99th=[24448]
lat (usec) : 10=3.46%, 20=12.54%, 50=26.66%, 100=41.16%, 250=13.64%
lat (usec) : 500=0.02%, 750=0.03%, 1000=0.01%
lat (msec) : 2=0.23%, 4=0.50%, 10=0.73%, 20=0.91%, 50=0.12%
cpu : usr=0.02%, sys=0.42%, ctx=299540, majf=0, minf=0
IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
issued : total=r=0/w=26955/d=0, short=r=0/w=0/d=0, drop=r=0/w=0/d=0
latency : target=0, window=0, percentile=100.00%, depth=1
Run status group 0 (all jobs):
WRITE: io=431280KB, aggrb=7186KB/s, minb=7186KB/s, maxb=7186KB/s,
mint=60015msec, maxt=60015msec
Disk stats (read/write):
nvme1n1: ios=0/30627, merge=0/2125, ticks=0/410990, in_queue=410990,
util=99.94%

Fio output (5.10.162 with revert):
write: io=441920KB, bw=7363.7KB/s, iops=460, runt= 60014msec
clat (usec): min=6, max=35768, avg=289.09, stdev=1736.62
lat (usec): min=6, max=35768, avg=289.28,stdev=1736.62
clat percentiles (usec):
| 1.00th=[ 8], 5.00th=[ 10], 10.00th=[ 16], 20.00th=[ 24],
| 30.00th=[ 36], 40.00th=[ 46], 50.00th=[ 59], 60.00th=[ 71],
| 70.00th=[ 83], 80.00th=[ 97], 90.00th=[ 110], 95.00th=[ 117],
| 99.00th=[10048], 99.50th=[14144], 99.90th=[21632], 99.95th=[25984],
| 99.99th=[28288]
lat (usec) : 10=4.13%, 20=11.67%, 50=26.59%, 100=39.57%, 250=15.28%
lat (usec) : 500=0.03%, 750=0.03%, 1000=0.03%
lat (msec) : 2=0.20%, 4=0.64%, 10=0.80%, 20=0.86%, 50=0.18%
cpu : usr=0.01%, sys=0.43%, ctx=313909, majf=0, minf=0
IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
issued : total=r=0/w=27620/d=0, short=r=0/w=0/d=0, drop=r=0/w=0/d=0
latency : target=0, window=0, percentile=100.00%, depth=1
Run status group 0 (all jobs):
WRITE: io=441920KB, aggrb=7363KB/s, minb=7363KB/s, maxb=7363KB/s,
mint=60014msec, maxt=60014msec
Disk stats (read/write):
nvme1n1: ios=0/31549, merge=0/2348, ticks=0/409221, in_queue=409221,
util=99.88%

Also i looked ext4_writepages latency which increases when the commit is
reverted. (This makes sense since the commit avoids unnecessary
transactions).

./funclatency ext4_writepages -->(5.10.162)
avg = 7734912 nsecs, total: 134131121171 nsecs, count: 17341

./funclatency ext4_writepages -->(5.10.162 with revert)
avg = 9036068 nsecs, total: 168956404886 nsecs, count: 18698

Looking at the journal transaction data I can see that the average
transaction commit time decreases after reverting the commit.
This probably helps in the IOPS score.

5.10.162:
cat /proc/fs/jbd2/nvme1n1-8/info --> After 1st test iteration
2143 transactions (2143 requested), each up to 8192 blocks
average:
0ms waiting for transaction
0ms request delay
0ms running transaction
0ms transaction was being locked
0ms flushing data (in ordered mode)
20ms logging transaction
20731us average transaction commit time
51 handles per transaction
1 blocks per transaction
3 logged blocks per transaction

cat /proc/fs/jbd2/nvme1n1-8/info/ --> After 2nd test iteration
4292 transactions (4292 requested), each up to 8192 blocks
average:
0ms waiting for transaction
0ms request delay
0ms running transaction
0ms transaction was being locked
0ms flushing data (in ordered mode)
20ms logging transaction
26470us average transaction commit time
51 handles per transaction
1 blocks per transaction
3 logged blocks per transaction

5.10.162 with revert:
cat /proc/fs/jbd2/nvme1n1-8/info--> After 1st test iteration
2092 transactions (2091 requested), each up to 8192 blocks
average:
0ms waiting for transaction
0ms request delay
20ms running transaction
0ms transaction was being locked
0ms flushing data (in ordered mode)
20ms logging transaction
15981us average transaction commit time
67 handles per transaction
1 blocks per transaction
2 logged blocks per transaction

/cat /proc/fs/jbd2/nvme1n1-8/info/*//--> After 2nd test iteration
4263 transactions (4262 requested), each up to 8192 blocks
average:
0ms waiting for transaction
0ms request delay
10ms running transaction
0ms transaction was being locked
0ms flushing data (in ordered mode)
20ms logging transaction
19795us average transaction commit time
65 handles per transaction
1 blocks per transaction
2 logged blocks per transaction

Looking at the commit it seems we should be avoiding unnecessary journal
transactions. This is reflected in the ext4_writepages latency.
But the transaction commit time seems to be increasing with this commit
leading to reduced IOPS. (atleast that's my theory).
Can you help look into why this commit introduces this IOPS regression?
Also any suggestions on running any more tests to isolate the issue are
welcome.

Thanks,
Rishabh