[OpenAFS-devel] RX: why retain soft acked packets at the sender?

[Jeffrey Altman]

This is a cryptographically signed message in MIME format.

--------------ms030700050609070502010209
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Rainer Toebbicke wrote:
> Looking at some of the RX performance problems that we see in particular 
> with respect to single stream throughput I stumbled across the following:
> 
> upon receipt of an ACK the sender retains all ACKed packets above 
> [t]first in the transmission queue (i.e. until they're hard acked).
> 
> The question is: why? The packet has been received by the RX stack at 
> the receiver, albeit not delivered to the "consumer". There is certainly 
> a point throttling transfer at the sender for congestion, hence 
> transmission and congestion windows, but why would the sender ever need 
> to resend a packet to a receiver who's already got it?
> 
> And if it could... what would be the mechanism? I only see the 
> possibility of explicitly nacking the packet later again, but unless the 
> receiver would drop a packet already received this can't be as far as I 
> can see, and I don't see the latter happening. Am I struck with 
> blindness or is this ancient history?
> 
> Given that long queues in conjunction with big window sizes are a drag I 
> plan to short-circuit this. Any comments?

I believe the existing behavior is most likely an artifact.  The ACK'd 
packet might have been held onto in order to ease debugging or because 
analysis of the code at the time indicated that frequently obtaining the 
global free packet queue mutex was a greater expense than holding onto 
the packets until an explicit ACK permitted multiple packets to be freed 
at once.   Now that we have per thread free packet queues reducing the 
lengths of the queues will be a win.

Clearly any patch will have to be reviewed and be tested.  Profiling of 
the code with and without the change would be useful to determine what 
benefits are achieved.

Jeffrey Altman

--------------ms030700050609070502010209
Content-Type: application/x-pkcs7-signature; name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7s"
Content-Description: S/MIME Cryptographic Signature
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--------------ms030700050609070502010209--