Re: [PATCH v2] net-tcp: TCP/IP stack bypass for loopback connections

On Tue, 14 Aug, Bruce Curtis wrote:

On Mon, Aug 13, 2012 at 11:31 PM, Bill Fink [off-list ref] wrote:

quoted

On Thu,  9 Aug 2012, Bruce "Brutus" Curtis wrote:

quoted

From: "Bruce \"Brutus\" Curtis" <redacted>

TCP/IP loopback socket pair stack bypass, based on an idea by, and
rough upstream patch from, David Miller [off-list ref] called
"friends", the data structure modifcations and connection scheme are
reused with extensive data-path changes.

A new sysctl, net.ipv4.tcp_friends, is added:
  0: disable friends and use the stock data path.
  1: enable friends and bypass the stack data path, the default.

The following is from a user perspective, since I am not
intimately familiar with the internals of the TCP stack.

I think tcp_friends is a poor name from a user POV.
Something like tcp_bypass would be much better.

I also believe it should be disabled by default, as that is
the current behavior, and those who would gain an advantage
from using it can easily enable it.

Changing the behavior would violate the principle of least
surprise.  Loopback TCP testing of an application or system
is often a useful first step in evaluating its behavior and
performance.  If the TCP stack is bypassed, it will give a
very false impression when such tests are performed.

Does it preserve all TCP semantics for applications, including
things like urgent data, ancillary data, and TCP socket options
and ioctls.  If it doesn't, it shouldn't be the default, and it
should be documented what features do and don't work when
tcp_bypass is enabled.  If all TCP semantics are unchanged,
that would also be good to know and document.

And there's the already mentioned issue of breaking tcpdump
and related tools.

While this could be a very useful feature in some environments,
it seems to me it would be safest to have it disabled by default.

                                        -Bill

1) tcp_friends vs tcp_bypass, the average user will not need to know
about this tunable so if there's consensus that it needs to be
changed, change it?

I see no reason to make it obtuse rather than something more
descriptive of its function (as opposed to how it's implemented).

2) this is a throughput/latency advantage for most (all?) so it
benefits most (all?) production environments

I grant that given that (4) below is true.

3) as for breaking tcpdump and ... Again, it does maintain the
connection establishment and finish packet flow so for most TCP
connection related interpose uses this should work and be documented
but if your trying to debug TCP's protocol state-machine, network
emulation, ... then Yes a user would need to disable but IMHO this is
the exception

4) all TCP socket semantics are maintained and if not it's a bug and
needs to be fixed

This was my biggest concern if it wasn't true.  Since you have now
verified that all TCP semantics are preserved, I now don't have a
major issue with it being enabled by default, since it's easy to
disable for more specialized situations.

I do have some concern that since the loopback path through the
TCP stack won't be heavily exercised anymore, it may be more likely
for bugs or performance degradations to creep into that code.

						-Thanks

						-Bill