Re: inode rwlock instead of semaphore

[Andrew Morton <akpm@digeo.com>]

Matthew Wilcox <willy@debian.org> wrote:
>
> On Sun, Feb 02, 2003 at 10:01:55AM -0700, Andreas Dilger wrote:
> > I'm wondering why we use a semaphore to lock directories on lookups instead
> > of a rwlock?  This would allow parallel lookups on directory entries instead
> > of single threading.  We have a need for directories with millions of files
> > in them, and being able to start parallel lookups would be a big performance
> > boost I think.
> 
> You mean a rwsem, not a rwlock, I assume?  How about starvation issues?
> 

Well...  things like starvation we could presumably fix with a new lock type
or whatever.

But Andreas is correct - holding i_sem on a directory while the holder is
performing synchronous I/O is a very serious scalability problem.

I hit it (badly) against /tmp: one process was unlinking a file in /tmp (and
hence waiting on underway writeback in truncate).  This prevents everything in
the machine from creating new files in /tmp until that I/O completes.

I fixed that specific problem by running the actual truncate outside i_sem in
sys_unlink().  But surely there will be other such problems.  sync and
dirsync mounts come to mind, as well as huge directories.