Re: [RFC v2] tcp: add support for read with offset when using MSG_PEEK

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 5 Sep 2023 12:16:11 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, Sep 04, 2023 at 01:14:17PM -0400, Jon Maloy wrote:
> > 
> > 
> > On 2023-06-23 07:02, Stefano Brivio wrote:  
> > > On Thu, 22 Jun 2023 22:12:27 -0400
> > > Jon Maloy <jmaloy@redhat.com> wrote:
> > >   
> > > > When reading received messages with MSG_PEEK, we sometines have to read
> > > > the leading bytes of the stream several times, only to reach the bytes
> > > > we really want. This is clearly non-optimal.
> > > > 
> > > > What we would want is something similar to pread/preadv(), but working
> > > > even for tcp sockets. At the same time, we obviously don't want to add
> > > > any new arguments to the recv/recvmsg() calls.
> > > > 
> > > > In this commit, we allow the user to set iovec.iov_base in the first
> > > > vector entry to NULL. This tells the socket to skip the first entry,
> > > > hence making the iov_len field of that entry indicate the offset value.
> > > > This way, there is no need to add any new arguments.  
> > > Ah-ha! I'm glad you found an acceptable way to pass a NULL pointer
> > > there. :)
> > >   
> > > > This change is simple and non-intrusive, and should be safe addition to
> > > > the socket API. We have measured it to give a throughput improvement of  
> > > ...it would be nice to also do a bit of profiling with perf(1) --
> > > that's where I originally noticed we were wasting cycles on filling up
> > > tcp_buf_discard. Plus, sure, there's also some value in dropping a
> > > useless 16 MiB buffer.
> > > 
> > > If you need examples/inspiration: the pasta (automated) demo shows
> > > that, skip at 9:20 in: https://passt.top/passt/about/#pasta_2 (the one
> > > on the left)
> > > 
> > > and that's simply done like this:
> > >    https://passt.top/passt/tree/test/demo/pasta#n163  
> > This looks like a script language, but I don't recognize it. How do I run
> > it?  
> 
> It's the hand rolled script language of the passt tests.  The
> interpreter is in test/lib/test.  Easiest way to run it is probably to
> "make check" in test/ of the passt tree.  If you want to selectively
> run certain things, edit test/run.

...some more details at test/README.md.

-- 
Stefano