On 1/13/23 2:13 PM, Bhatnagar, Rishabh wrote:
On 1/12/23 3:38 AM, Jan Kara wrote:CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.There is a difference of 5% with and with the commit if i change this to fdatasync=1.Hi! On Wed 11-01-23 18:06:39, Bhatnagar, Rishabh wrote:We have been seeing a consistent 3% degradation in IOPS score between 5.4and 5.10 stable kernels while running fio tests.I'm running test case on m6g.8xlarge AWS instances using arm64. The testinvolves: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=${filesize} \ --fsync=1 --group_reporting --create_only=1 > /dev/null sudo echo 1 > /proc/sys/vm/drop_caches set -xecho "Running with jobs=${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 base5.10 kernel.Although the average clat is higher after reverting the commit the higher bwdrives the iops score higher as seen in below fio output.I expect the difference is somewhere in waiting for the journal. Can youjust check whether there's a difference if you use --fdatasync=1 instead of--fsync=1? With this workload that should avoid waiting for the journal because the only metadata updates are mtime timestamps in the inode.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=0IO 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=0IO 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 isreverted. (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.So what the workload is doing is: write() inode lock dirty 16k of page cache dirty inode i_mtime inode unlock fsync() walk all inode pages, write dirty ones wait for all pages under writeback in the inode to complete IO force transaction commit and wait for it Now this has the best throughput (and the worst latency) if all 16 processes work in lockstep - i.e., like: task1 task2 task3 ... write() write() write() fsync() fsync() fsync()because in that case we writeout all dirty pages from 16 processes in onesweep together and also we accumulate 16 mtime updates in single transaction commit. Now I suspect the commit you've identified leads to less synchronization between the processes and thus in less batching happening. In particularbefore the commit we've called mpage_prepare_extent_to_map() twice and the second invocation starts at where the first invocation saw last dirty page. So it potentially additionally writes newly dirtied pages beyond that placeand that effectively synchronizes processes more.To confirm the theory, it might be interesting to gather a histogram of a number of pages written back by ext4_writepages() call with / without thecommit. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CRHi JanI collected some data w.r.t to number of pages being written by ext4_writepages. What you pointed out seems to be correct. Without the commit I see more batching (more writeback count from 4-20 pages)happening compared to with the commit.Without the commit (reverted): [0-1] —> 4246 [2-3] —> 312 [4-5] —> 20836 [6-7] —> 205 [8-9] —> 895 [10-11] —> 56 [12-13] —> 422 [14-15] —> 62 [16-17] —> 234 [18-19] —> 66 [20-21] —> 77 [22-23] —> 9 [24-25] —> 26 [26-27] —> 1 [28-29] —> 13 Average page count : 3.9194 With the commit: [0-1] —> 1635 [2-3] —> 123 [4-5] —> 24302 [6-7] —> 38 [8-9] —> 604 [10-11] —> 19 [12-13] —> 123 [14-15] —> 12 [16-17] —> 24 [18-19] —> 3 [20-21] —> 8 [22-23] —> 1 [24-25] —> 3 [26-27] —> 0 [28-29] —> 1 Average page count : 3.9184Also looking at journal data I see that without the commit we have more handles per journal transaction:Without the commit: cat /proc/fs/jbd2/nvme1n1-8/info 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 With the commit: cat /proc/fs/jbd2/nvme1n1-8/info 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 Thanks Rishabh
Hi Jan Gentle reminder.Any thoughts on this data? Is there a usecase where this commit brings benefits that are worth considering if we decide to revert this?
