netdev - Re: [PATCH v2] sctp: be more restrictive in transport selection on bundled sacks

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Date:	Thu, 28 Jun 2012 11:33:08 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	linux-sctp@...r.kernel.org
Subject: Re: [PATCH v2] sctp: be more restrictive in transport selection on
 bundled sacks

On Wed, Jun 27, 2012 at 03:44:22PM -0400, Vlad Yasevich wrote:
> On 06/27/2012 01:28 PM, Neil Horman wrote:
> >On Wed, Jun 27, 2012 at 11:10:26AM -0400, Vlad Yasevich wrote:
> >>
> >>I didn't think of this yesterday, but I think it would be much
> >>better to use pkt->transport here since you are adding the chunk to
> >>the packet and it will go out on the transport of the packet.  You
> >>are also guaranteed that pkt->transport is set.
> >>
> >I don't think it really matters, as the chunk transport is used to lookup the
> >packet that we append to, and if the chunk transport is unset, its somewhat
> >questionable as to weather we should bundle, but if packet->transport is set,
> >its probably worth it to avoid the extra conditional.
> >
> 
> Just looked at the code flow.  chunk->transport may not be set until
> the end of sctp_packet_append_chunk.  For new data, transport may
> not be set.  For retransmitted data, transport is set to last
> transport data was sent on.  So, we could be looking at the wrong
> transport.  What you are trying to decided is if the current
> transport we will be used can take the SACK, but you may not be
> looking at the current transport. Looking at packet->transport is
> the correct thing to do.
> 
> -vlad
> 
So, I agree after what you said above, that this is the right thing to do.  That
said, I just tested the change with the SCTP_RR test in netperf, and it wound up
giving me horrid performance (Its reporting about 5 transactions per second).
It appears that whats happening is that, because the test alternates which
transports it sends out, and because it waits for a sack of teh prior packet
before it sends out the next transaction, we're always missing the bundle
opportunity, and always waiting for the 200ms timeout for the sack to occur.
While I know this is a pessimal case, it really seems bad to me.  It seems that
because I was using chunk->transport previously, I luckily got the transport
wrong sometimes, and it managed to bundle more often.

So I'm not sure what to do here.  I had really wanted to avoid adding a sysctl
here, but given that this is likely a corner cases, it seems that might be the
best approach.  Do you have any thoughts?

Neil

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