[erlang-questions] why is gen_tcp:send slow?

Per Hedeland <>
Wed Jun 25 22:24:48 CEST 2008


Johnny Billquist <> wrote:
>
>Per Hedeland skrev:
>> 
>> Whether this is Edwin's problem I don't know - the "fixed packet rate"
>> observation may actually be more or less correct: As you explained, TCP
>> doesn't ack packets, it acks bytes - but the actual *sending* of acks is
>> definitely related to the reception of packets (or "segments" if you
>> prefer), in particular in a one-way data transfer where there are no
>> outgoing data packets that can have acks "piggy-backed". The details may
>> vary, but in general in such a case an ack is sent for every other
>> packet received, or after a ("long" - 200 ms) timeout if no packets are
>> received.
>
>Hmm. Well, a tcpdump will quickly tell if the window size is zero, and he's 
>hitting the RTT-limiting factor.

Hmm hmm, I think I wasn't thinking straight when I wrote the above - I
was speculating that this could be something other than the RTT-limiting
factor (otherwise small vs big packets shouldn't make a difference in
throughput), along the lines of "the receiver has data to ack but is
delaying the sending of the ack until he recieves another segment".  But
this can't be the explanation I believe - if you're sending smaller
packets, you can send that many more into a given window before you need
to wait for an ack, so there are more "opprtunities" for the reciever to
send a delayed ack.

>Oh well, someone needs to look into this a bit more obviously. I haven't even 
>properly looked at the problem description. I just thought I'd point out some 
>wrong assumptions on how TCP works. :-)

Agreed!

--Per



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