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

5 Sep 2023


      On Tue, 5 Sep 2023 12:16:11 +1000
David Gibson  wrote:
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On Mon, Sep 04, 2023 at 01:14:17PM -0400, Jon Maloy wrote:
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On 2023-06-23 07:02, Stefano Brivio wrote:
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On Thu, 22 Jun 2023 22:12:27 -0400
Jon Maloy  wrote:
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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