[Samba] Samba write performance in kernel

[Jeremy Allison]

On Tue, Aug 26, 2008 at 05:41:51PM +0800, Lin Mac wrote:
> 
> hi,
> 
> I would like to know is it possible to make writing file to samba completely in kernel?

Yes.

> I'm using a slow CPU (FA526) , and the memory copy is even slower. The reading performance is over 7 MB/s, with mmap and sendfile enabled, while writing is only 4-5 MB/s. Without mmap and sendfile, reading from samba is also about 4-5 MB/s. I use Oprofile to profile writing file to samba and found that CPU takes over 30% CPU time on copy_from/to_user, so I think going to user space and back again is the bottleneck.
> 
> Since there is sendfile, why is'nt there counterpart on write path? Is there some difficalties or what? Is it implementable?

There is a counterpart on the write path, called RECEIVEFILE
in the Samba 3.2.x code. It's already coded up in userspace,
just waiting the kernel counterpart :-). If there's no kernel
receivefile() call then it emulates this in userspace.

Try plumbing in the Linux splice() system call (this is
already coded up in Samba, awaiting working kernel support).

Jeremy.