This patch provides single stream write performance improvement with multiple worker threads in llite layer. its operation overvies is the following

In system call context
1) get a worker thread's lu_env
2) assemble and set parameters
2-1) copy user buffer to kernel buffer
2-2) copy parameters needed for worker thread to resume I/O
2-3) set parameters to the lu_env gotten in (1)
2-4) set extra parameters to an I/O item, iosvc_item
3) inform worker thread: ll_iosvc, we got ready.
4) return immediately

In worker thread context
1) wake up
2) gathering information
2-1) refer its own lu_env to know the parameters set by syscall
2-2) refer the item made in (2-3)
3) resume I/O
4) sleep

I attached the performance to compare the original Lustre-2.7.52 and this custom Lustre-2.7.52