On Tue, 7 Mar 2006 00:34:38 +0200
"Michael S. Tsirkin" <[email protected]> wrote:
> Hello, Dave!
> As you might know, the TSO patches merged into mainline kernel
> since 2.6.11 have hurt performance for the simple (non-TSO)
> high-speed netdevice that is IPoIB driver.
>
> This was discussed at length here
> http://openib.org/pipermail/openib-general/2005-October/012271.html
>
> I'm trying to figure out what can be done to improve the situation.
> In partucular, I'm looking at the Super TSO patch
> http://oss.sgi.com/archives/netdev/2005-05/msg00889.html
>
> merged into mainline here
>
> http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=314324121f9b94b2ca657a494cf2b9cb0e4a28cc
>
> There, you said:
>
> When we do ucopy receive (ie. copying directly to userspace
> during tcp input processing) we attempt to delay the ACK
> until cleanup_rbuf() is invoked. Most of the time this
> technique works very well, and we emit one ACK advertising
> the largest window.
>
> But this explodes if the ucopy prequeue is large enough.
> When the receiver is cpu limited and TSO frames are large,
> the receiver is inundated with ucopy processing, such that
> the ACK comes out very late. Often, this is so late that
> by the time the sender gets the ACK the window has emptied
> too much to be kept full by the sender.
>
> The existing TSO code mostly avoided this by keeping the
> TSO packets no larger than 1/8 of the available window.
> But with the new code we can get much larger TSO frames.
>
> So I'm trying to get a handle on it: could a solution be to simply
> look at the frame size, and call tcp_send_delayed_ack from
> if the frame size is no larger than 1/8?
>
> Does this make sense?
>
> Thanks,
>
>
More likely you are getting hit by the fact that TSO prevents the congestion
window from increasing properly. This was fixed in 2.6.15 (around mid of Nov 2005).
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