fio error if numjobs > 15, got signal=7

[lcxywfe]

root@029c49d66f62:~/fio# ./fio -numjobs=16 -fallocate=none
-ioengine=external:/root/3fs/benchmarks/fio_usrbio/hf3fs_usrbio.so
-direct=1
-rw=write
-bs=4MB
--group_reporting
-filesize=500MB
--nrfiles=10
-iodepth=1
-name=/3fs/stage/test0/test
-mountpoint=/3fs/stage
-ior_depth=1
/3fs/stage/test0/test: (g=0): rw=write, bs=(R) 4096KiB-4096KiB, (W) 4096KiB-4096KiB, (T) 4096KiB-4096KiB, ioengine=hf3fs_usrbio, iodepth=1
...
fio-3.40
Starting 16 processes
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
/3fs/stage/test0/test: Laying out IO files (10 files / total 5000MiB)
fio: pid=7379, got signal=7
bs: 15 (f=150): [W(8),K(1),W(7)][2.8%][w=360MiB/s][w=90 IOPS][eta 03m:30s]

if numjobs>15, then will get "fio: pid=7379, got signal=7"