Re: [RFC PATCH net-next 0/3 V4] net-tcp: TCP/IP stack bypass for loopback connections

On 12/11/2012 05:02 AM, David Miller wrote:

From: Weiping Pan<redacted>
Date: Wed,  5 Dec 2012 10:54:16 +0800

quoted

Friends VS AF__UNIX
Their call path are almost the same, but AF_UNIX uses its own send/recv codes
with proper locks,
so AF_UNIX's performance is much better than Friends.

Sorry, this statement is not correct.
In TCP_STREAM case, if the message size if 16384, then AF_UNIX is much better than Friends.
If the message size is smaller, then Friends shows equal performance with AF_UNIX.
In TCP_RR,  Friends shows equal performance with AF_UNIX, too.

While I understand the other portions of your analysis, this one
mystifies me.

In both cases, the sender has to queue the SKB onto the receiver's
queue.  And in both cases, the sender takes the lock on that queue.

So the locking contention really ought to be similar if not identical.

The only difference is that AF_UNIX takes the unix_sk()->lock of the
remote socket around these operations.

If that is enough of a synchronizer to "fix" the contention or reduce
it, then this would be very easy to test by adding a friend lock to
tcp_sk().

I make some experiments to reduce the use of lock,
some performance results will be followed up.

thanks
Weiping Pan

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