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Re: [PATCH] Improve behaviour of Netlink Sockets

To: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH] Improve behaviour of Netlink Sockets
From: jamal <hadi@xxxxxxxxxx>
Date: 28 Sep 2004 08:32:17 -0400
Cc: Pablo Neira <pablo@xxxxxxxxxxx>, "David S. Miller" <davem@xxxxxxxxxx>, netdev@xxxxxxxxxxx
In-reply-to: <20040928111159.GA18421@xxxxxxxxxxxxxxxxxxx>
Organization: jamalopolous
References: <20040924032440.GB6384@xxxxxxxxxxxxxxxxxxx> <1096289189.1075.37.camel@xxxxxxxxxxxxxxxx> <20040927213607.GD7243@xxxxxxxxxxxxxxxxxxx> <1096339407.8660.33.camel@xxxxxxxxxxxxxxxx> <20040928024614.GA9911@xxxxxxxxxxxxxxxxxxx> <1096340772.8659.51.camel@xxxxxxxxxxxxxxxx> <20040928032321.GB10116@xxxxxxxxxxxxxxxxxxx> <1096343125.8661.96.camel@xxxxxxxxxxxxxxxx> <20040928035921.GA10675@xxxxxxxxxxxxxxxxxxx> <1096367787.8662.146.camel@xxxxxxxxxxxxxxxx> <20040928111159.GA18421@xxxxxxxxxxxxxxxxxxx>
Reply-to: hadi@xxxxxxxxxx
Sender: netdev-bounce@xxxxxxxxxxx
On Tue, 2004-09-28 at 07:11, Herbert Xu wrote:
> On Tue, Sep 28, 2004 at 06:36:27AM -0400, jamal wrote:
> > 
> > er, what about the host scope route msgs generated by same script? ;->

> AFAIK, no async netlink event function uses NLM_GOODSIZE at all for
> obvious reasons.

Actually i just examined the events generated by the script, they are
IFLA and ROUTE events and not IFA.
So take a look at:rtmsg_ifinfo()

> > The state is per socket. You may need an intermediate queue etc which
> > feeds to each user socket registered for the event. The socket queue
> > acts as a essentially a retransmitQ for broadcast state. Just waving my
> > hands throwing ideas here of course.
> 
> Aha, you've fallen into my trap :) 

Oh, goody ;-> 

> Now let me demonstrate why having
> an intermediate queue doesn't help at all.
> 
> Holding the packet on the intermediate queue is exactly the same as
> holding it on the receive queue of the destination socket.  The reason
> is that we're simply cloning the packets.  So moving it from one queue
> to another does not reduce system resource usage by much.

Ah, but theres clearly benefit into saving packets from crossing to user
space in particular in the case of overload. You do save on system
resources for sure in that case.
In the case of normal operation, no overload case, you end up using a
little more system resource - but thats a price tag that comes with the
benefits (needs to be weighed out).

> There is the cost in cloning the skbs.  However, that's an orthogonal
> issue altogether.  We can reduce the cost there by making the packets
> bigger.  This can either be done at the sender end by coalescing
> successive messages.  Or we can merge them in netlink_broadcast.
> 
> Granted having an intermediate queue will avoid overruns if it is
> large enough.  However, having all the receive queues to be as big
> as your intermediate queue will have exactly the same effect.

Agreed it will postpone the problem, and not cure it.
Where i saw the benefit is if this queue is full/overloaded then you
dont bother transfering skbs to the sock receiveQ - instead you overrun
the event listeners (on purpose) before giving them any data. This
assumes you only start feeding the listeners when the event generation
is complete (in multi message batch when DONE is processed).
In the case of overrunning the listeners you should alos flush the
intermediate queue.
Again, I am handwaving - there maybe a lot of practical issues whcih
become obvious with actually get hands dirty. I actually tried to
implement this a while back for socket packet tap listeners; cant find
my patches. What i was trying to get feedback that i could feed all the
way down to NAPI - it provide to be futile because i would need to have
hardware drop selectively and such NICs dont exist.

> In fact this has an advantage over the intermediate queue.  With the
> latter, you need to hold the packet in place whether the applications
> need it or not.  While currently, the application can choose whether
> it wants to receive a large batch of events and if so how large.
> 

Right, but only find out after reading a subset of messages which cross
into user space. Which is wasted cycles really.
Now if you could say from user space "please continue where you left
over" the messages before overrun wont be a waste. I do think thats not
wise for events(you should be able a summary of the issue some other way
as in overruns at the moment) but is definetely need for large get
results.

> Remember just because one application overruns, it doesn't mean that
> the other recipients of the same skb will overrun.  They can continue
> to receive messages as long as their receive queue allows it.
> 

Agreed. Note thats a design choice and the truth of which is a better
scheme only comes out by testing both schemes. Easier to leave
whats already in place - but what fun is that now?;->
Also to note this only applies to broadcasts.

> So applications that really want to see every event should have a
> very large receive queue.  Those that can recover easily should use
> with a much smaller queue.

Depending on how you look at it (since i am drinking the write variant
of cofee right now, lets look at it from a philosphoical view: is it the
area of the light radiated or the circumference of darkness surrounding
the light? ;-> Choose your metric ;-> ):
A large queue may actually be a problem if it also gets overflown since
it takes relatively longer to find out. You still have to read the damn
state to find out details.
 
[..]

> Jamal, maybe I've got the wrong impression but it almost seems
> that you think that if one applications overruns, then everyone
> else on that multicast address will overrun as well.  This is
> definitely not the case.


I think its fair to assume that if the intermidiate queue is overflown
all listeners will be.

> With an intermediate queue, you will in fact impose overruns on
> everyone when it overflows which seems to be a step backwards.

Refer to above assumption.

> > The moral of this is: you could do it if you wanted - aint trivial.
> 
> Well this is not what I'd call congestion control :) Let's take a
> TCP analogy.  This is like batching up TCP packets on a router in
> the middle rather than shutting down the sender.  Congestion control
> is where you shut the sender up.

Its actually worse than that -->which is a shame since we have more
control over what can be sent to user.
Congestion could be driven by receiver as well. Look at TCP zero windows
for example. Or even ECN.

cheers,
jamal


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