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Date: Fri, 14 Jun 2013 17:39:12 +0530
Message-ID: <CAEfL3Km7knMAW1Jx_jHZ0OYBMBpUkvbzk2riBE2C=NA9OMvUQw@mail.gmail.com>
Subject: Re: why does nfsd write not use splice
From: Sandeep Joshi <sanjos100@gmail.com>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: linux-nfs@vger.kernel.org
Content-Type: text/plain; charset=ISO-8859-1
Sender: linux-nfs-owner@vger.kernel.org

On Wed, Jun 12, 2013 at 10:16 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
>
> On Wed, Jun 12, 2013 at 09:51:09PM +0530, Sandeep Joshi wrote:
> > Splice can be implemented independent of RDMA.  It is supposed to
> > transfer
> > pages between two file descriptors.  I found some postings on lkml from
> > 2006 where Linus says it is quite possible to splice from a socket to a
> > file.
> >
> > See the paragraph:
> > " For filesystems, splice support tends to be really easy (both read and
> > write). For other things, it depends a bit. But unlike sendfile(), it
> > really is quite possible to splice _from_ a socket too, not just _to_ a
> > socket. But no, that case hasn't been written yet."
> >  http://yarchive.net/comp/linux/splice.html
> >
> > Larry McVoy's 1997 proposal for adding splice support to the  kernel can
> > be
> > read at
> > ftp.tux.org/pub/sites/ftp.bitmover.com/pub/*splice*.*ps*.gz<http://ftp.tux.org/pub/sites/ftp.bitmover.com/pub/splice.ps.gz>
> >
> > Perhaps I should have opened this thread on lkml to determine if splice
> > from socket to file is still feasible..
>
> Right, the thing is, nfsd reads the rpc request from the socket into its
> own buffers before it parses it.  If you want to move the data directly
> out of the network buffers into the page cache, then you have to know at
> what point the write data starts in the request--which I believe will
> mean doing the xdr parsing (and gss decryption if necessary) as the
> request comes in off the wire.
>
> That sounds like a lot of work and even if you have someone willing to
> do the work they'd also need to justify that it's worth it.
>
> RDMA may have some protocol support that simplifies this, I don't know.
>
> --b.

Hi Bruce,

> nfsd reads the rpc request from the socket into its own buffers before it parses it.

I am not intimate with the gss code but do you think the
svc_rqst->rq_pages[] can be spliced ?

-Sandeep