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

["Jeff V. Merkey" <jmerkey@timpanogas.org>]

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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