netdev - RE: [PATCH v2 0/2] net/sctp: Avoid allocating high order memory with kmalloc()

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Date:   Mon, 6 Aug 2018 09:34:05 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Michael Tuexen' <Michael.Tuexen@...chi.franken.de>,
        "Marcelo Ricardo Leitner" <marcelo.leitner@...il.com>
CC:     Konstantin Khorenko <khorenko@...tuozzo.com>,
        "oleg.babin@...il.com" <oleg.babin@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        Xin Long <lucien.xin@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: RE: [PATCH v2 0/2] net/sctp: Avoid allocating high order memory with
 kmalloc()

From: Michael Tuexen
> Sent: 03 August 2018 21:57
...
> >> Given how useless SCTP streams are, does anything actually use
> >> more than about 4?
> >
> > Maybe Michael can help us with that. I'm also curious now.
> In the context of SIGTRAN I have seen 17 streams...

Ok, I've seen 17 there as well, 5 is probably more common.

> In the context of WebRTC I have seen more streams. In general,
> the streams concept seems to be useful. QUIC has lots of streams.
> 
> So I'm wondering why they are considered useless.
> David, can you elaborate on this?

I don't think a lot of people know what they actually are.

Streams just allow some receive data to forwarded to applications when receive
message(s) on stream(s) are lost and have to be retransmitted.

I suspect some people think that the separate streams have separate flow control,
not just separate data sequences.

M2PA separates control message (stream 0) from user data (stream 1).
I think the spec even suggests this is so control messages get through when
user data is flow controlled off - not true (it would be true for ISO
transport's 'expedited data).

M3UA will use 16 streams (one for each (ITU) SLS), but uses stream 0 for control.
If a data message is lost then data for the other sls can be passed to the
userpart/mtp3 - this might save bursty processing when the SACK-requested
retransmission arrives. But I doubt you'd want to run M3UA on anything lossy
enough for more than 4 data streams to make sense.

Even M3UA separating control onto stream 0 data onto 1-n doesn't seem useful to me.

If QUIC is using 'lots of streams' is it just using the stream-id as a qualifier
for the data? Rather than requiring the 'not head of line blocking' feature
of sctp streams?

Thought....
Could we let the application set large stream-ids, but actually mask them
down to (say) 32 for the protocol code?

	David

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