Re: Negative scalability by removal of lock_kernel()?(Was: Strange performance behavior of 2.4.0-test9)

Jeff V. Merkey
2000-10-27T06:36:45Z

Linux has lots of n-sqared linear list searches all over the place, and
there's a ton of spots I've seen it go linear by doing fine grained
manipulation of lock_kernel() [like in BLOCK.C in NWFS for sending async
IO to ll_rw_block()]. I could see where there would be many spots
where playing with this would cause problems.

2.5 will be better.

Jeff

kumon@flab.fujitsu.co.jp wrote:
>
> Finally, I found:
> Removal of lock_kernel in fs/fcntl.c causes the strange performance of
> 2.4.0-test9.
>
> The removal causes following negative scalability on Apache-1.3.9:
> * 8-way performance dropped to 60% of 4-way performance.
> * Adding lock_kernel() gains 2.4x performance on 8-way.
>
> This suggests some design malfunction exist in the fs-code.
>
> The lock_kernel() is removed in test9, as shown in below, then the
> strange behavior appeared.
>
> linux-2.4.0-test8/fs/fcntl.c:
> asmlinkage long sys_fcntl(unsigned int fd, unsigned int cmd, unsigned long arg)
> {
> struct file * filp;
> long err = -EBADF;
>
> filp = fget(fd);
> if (!filp)
> goto out;
>
> --> lock_kernel();
> err = do_fcntl(fd, cmd, arg, filp);
> --> unlock_kernel();
>
> fput(filp);
> out:
> return err;
> }
>
> Adding the lock_kernel()/unlock_kernel() to test9:fs/fcntl.c,
> The performance is restored,
> The number of task switch is reduced, and
> Positive scalability is observed.
>
> The lock region may be narrowed to around call of posix_lock_file()
> in fcntl_setlk() (fs/locks.c).
>
> I usually prefer removal of kernel_lock, but at this time,
> the removal severy struck the performance.
>
> Please give me suggestions..
>
> kumon@flab.fujitsu.co.jp writes:
> > kumon@flab.fujitsu.co.jp writes:
> > > Rik van Riel writes:
> > > > On Wed, 25 Oct 2000 kumon@flab.fujitsu.co.jp wrote:
> > > > > I found very odd performance behavior of 2.4.0-test9 on a large SMP
> > > > > server, and I want some clues to investigate it.
> > > > >
> > > > > 1) At the 8 cpu configuration, test9 shows extremely inferior
> > > > > performance.
> > > > > 2) on test8, 8-cpu configuration shows about 2/3 performance of 4-cpu.
> > > > ^^^^^ test9 ??
> >
> > IMHO, the modification of file-system code causes the weird
> > performance.
> >
> > Most of processes are slept at:
> > posix_lock_file()->locks_block_on()->interruptible_sleep_on_locks()
> >
> > We revert two of test9 files (fs/fcntl.c fs/flock.c), to the previous
> > version, the performance problem disappeared and it becomes to the
> > same level as test8.
> >
> > To narrow the problem, we measured performance of 3 configuration:
> > 1) test9 with test8 fs/fcntl.c, test8 fs/flock.c
> > 2) test9 with test8 fs/fcntl.c
> > 3) test9 with test8 fs/flock.c
> >
> > Only 3) shows the problem, so the main problem reside in fcntl.c (not
> > in flock.c).
> >
> > So it seems:
> > the web-server, apache-1.3.9 in the redhat-6.1, issues lots of fcntl
> > to the file and those fcntls collide each other, and the processes
> > are blocked.
> >
> >
> > What has happend to fcntl.c?
> >
> > --
> > Computer Systems Laboratory, Fujitsu Labs.
> > kumon@flab.fujitsu.co.jp
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