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Harald Welte laforge at gnumonks.orgHi Holger, Pablo, Ciaby,
On Sat, Apr 04, 2015 at 09:05:20AM +0200, Holger Hans Peter Freyther wrote:
> * Linux/TCP will run nagle to combine these messages
we could easily disable nagle if that's what we want to avoid
latencies [see below]
> I wonder if there is a better way? For reading we could read
> until -EWOULDBLOCK but then this might not be too fair for
> the other parts of the software.
In terms of msgb reading, I think a fixed upper limit might make sense,
i.e. read up to [configurable] N messages in one BSC_FD_READ callback.
> For writing we might end up in the situation where a write only
> partially succeeds and we need to remember how much of the msgb to
> write next... So we would need to do ioctl's to check how much space
> is left in the send buffer
Even if you do the ioctl(SICOUTQ) / getsockopt(SO_SNDBUF), what would
you do next? i.e. if you have determined not to write the msgb due to
insufficient socket buffer space. If you then leave BSC_FD_WRITE set,
then your next select will immediately return, and you have a potential
to busy-loop until finally the buffer space is available. In case the
remote end / network link is stalled this would be quite dangerous.
If you don't set BSC_FD_WRITE, then you wouldn't notice once the buffer
has sufficient space again. If you did periodic checks, then the
latency introduced by those checks are probably counter-productive.
So I think we will have to deal with partial writes from the IPA msgb
sending code, just like the IPA msgb receiving code has to deal with
partial reads.
> Do you have ideas? comments? is it a non issue? is it something
> we can do better?
It depends probably on the use case. On expensive (e.g. sat) links you
probably want nagle to combine multiple packets. In all other cases
disabling nagle is probably a good idea to get started.
> Is there a TCP mode where a "write" either fully succeeds or fails
> with -ENOSPC or such?
Not to my knowledge. The proper answer would probably to use DCCP or
SCTP or any other protocol that can provide reliable delivery of packets
(possibly with ordering guarantees) and not abuse a stream protocol
like TCP. This would of course break IPA compatibility... but it might
be an interesting experiment to compare different L4 protocol
performance against TCP for the given use case.
--
- Harald Welte <laforge at gnumonks.org> http://laforge.gnumonks.org/
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