netdev - Re: [PATCH v3 0/2] sctp: delay calls to sk_data

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Date:	Thu, 14 Apr 2016 16:33:28 -0300
From:	marcelo.leitner@...il.com
To:	David Miller <davem@...emloft.net>
Cc:	nhorman@...driver.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.

I see. Okay, I'll revisit this, thanks.

  Marcelo