Message-ID: <20160414200351.GA4632@hmsreliant.think-freely.org>
Date:	Thu, 14 Apr 2016 16:03:51 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	David Miller <davem@...emloft.net>
Cc:	marcelo.leitner@...il.com, netdev@...r.kernel.org,
	vyasevich@...il.com, linux-sctp@...r.kernel.org,
	David.Laight@...LAB.COM, jkbs@...hat.com
Subject: Re: [PATCH v3 0/2] sctp: delay calls to sk_data_ready() as much as
 possible

On Thu, Apr 14, 2016 at 02:59:16PM -0400, David Miller wrote:
> From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> Date: Thu, 14 Apr 2016 14:00:49 -0300
> 
> > Em 14-04-2016 10:03, Neil Horman escreveu:
> >> On Wed, Apr 13, 2016 at 11:05:32PM -0400, David Miller wrote:
> >>> From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> >>> Date: Fri,  8 Apr 2016 16:41:26 -0300
> >>>
> >>>> 1st patch is a preparation for the 2nd. The idea is to not call
> >>>> ->sk_data_ready() for every data chunk processed while processing
> >>>> packets but only once before releasing the socket.
> >>>>
> >>>> v2: patchset re-checked, small changelog fixes
> >>>> v3: on patch 2, make use of local vars to make it more readable
> >>>
> >>> Applied to net-next, but isn't this reduced overhead coming at the
> >>> expense of latency?  What if that lower latency is important to the
> >>> application and/or consumer?
> >> Thats a fair point, but I'd make the counter argument that, as it
> >> currently
> >> stands, any latency introduced (or removed), is an artifact of our
> >> implementation rather than a designed feature of it.  That is to say,
> >> we make no
> >> guarantees at the application level regarding how long it takes to
> >> signal data
> >> readines from the time we get data off the wire, so I would rather see
> >> our
> >> throughput raised if we can, as thats been sctp's more pressing
> >> achilles heel.
> >>
> >>
> >> Thats not to say I'd like to enable lower latency, but I'd rather have
> >> this now,
> >> and start pondering how to design that in.  Perhaps we can convert the
> >> pending
> >> flag to a counter to count the number of events we enqueue, and call
> >> sk_data_ready every  time we reach a sysctl defined threshold.
> > 
> > That and also that there is no chance of the application reading the
> > first chunks before all current ToDo's are performed by either the bh
> > or backlog handlers for that packet. Socket lock won't be cycled in
> > between chunks so the application is going to wait all the processing
> > one way or another.
> 
> But it takes time to signal the wakeup to the remote cpu the process
> was running on, schedule out the current process on that cpu (if it
> has in fact lost it's timeslice), and then finally look at the socket
> queue.
> 
> Of course this is all assuming the process was sleeping in the first
> place, either in recv or more likely poll.
> 
> I really think signalling early helps performance.
> 

Early, yes, often, not so much :).  Perhaps what would be adventageous would be
to signal at the start of a set of enqueues, rather than at the end.  That would
be equivalent in terms of not signaling more than needed, but would eliminate
the signaling on every chunk.   Perhaps what you could do Marcelo would be to
change the sense of the signal_ready flag to be a has_signaled flag.  e.g. call
sk_data_ready in ulp_event_tail like we used to, but only if the has_signaled
flag isn't set, then set the flag, and clear it at the end of the command
interpreter.

That would be a best of both worlds solution, as long as theres no chance of
race with user space reading from the socket before we were done enqueuing (i.e.
you have to guarantee that the socket lock stays held, which I think we do).

Neil

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