[LU-12748] parallel readahead needs to be optimized at high number of process

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.14.0
• Environment: master
• Severity: 3
• Assignee: Wang Shilong (Inactive)
• Reporter: Shuichi Ihara

Description

parallel readahead is enabled by default in master, it contributes to sequential read performance a lot. 
However, if the number of IO thread is increased (e.g. NP=NCPU), read performance drops and it's lower than without readahead. it needs to be tunning and optimization.
Here is test configuration and resutls.

Client
2 x Platinum 8160 CPU @ 2.10GHz, 192GB memory, 2 x IB-EDR(multi-rail)
CentOS7.6(3.10.0-957.27.2.el7.x86_64)
OFED-4.5

for i in 6 12 24 48; do                                                                                                                                       
        size=$((768/i))                                                                                                                                       
        /work/tools/mpi/gcc/openmpi/2.1.1/bin/mpirun --allow-run-as-root -np $i /work/tools/bin/ior -w -r -t 1m -b ${size}g -e -F -vv -o /scratch0/file  | tee
 ior-1n${i}p-${VER}.log                                                                                                                                       
done

Summaruy of Read Performance(MB/sec)

branch	thr=6	thr=12	thr=24	thr=48
b2_12	9,965	14,551	17,177	18,152
master	15,252	16,026	17,842	16,991
master(pRA=off)	10,253	14,489	17,839	18,658

pRA=off  - disabling parallel readahead (llite.*.read_ahead_async_file_threshold_mb=0)

Issue Links

• is related to: LU-13138 sanity test 101d fails with 'readahead 283s > no-readahead 134s 500M'
• is related to: LU-13258 Bind linux workqueues to specific core